IN PRESS ARTICLE Remote Predictive Mapping 5. Using a Lidar Derived DEM to Test the Influence of Variable Overburden Thickness and Bedrock on Drainage and Basin Morphology Tim L. Webster, John C. Gosse, Ian Spooner, and J. Brendan Murphy DOI: http://www.dx.doi.org/10.12789/geocanj.2014.41.033 To appear in: Geoscience Canada Received: April 2012 Accepted as revised: July 2013 First published on the web: January 2014 Articles ‘In Press’ are peer-reviewed, accepted articles that have undergone layout in journal format, subsequent proof-editing by the author(s) and are ready for publication. The article in this form is the version of record. The article will be removed from this page and paginated once it is published as part of an issue. The date the article is first made available online as either “Accepted” or “In Press” will be carried forward to the published article. The article has been assigned a doi number that is finalized and citable. The doi number will be consistent with the article through to issue publishing and will be activated in CrossRef when the paper has been published in a Geoscience Canada issue.
Transcript
IN PRESS ARTICLE
Remote Predictive Mapping 5. Using a Lidar Derived DEM to Test the Influence of Variable Overburden Thickness and Bedrock on Drainage and Basin Morphology Tim L. Webster, John C. Gosse, Ian Spooner, and J. Brendan Murphy DOI: http://www.dx.doi.org/10.12789/geocanj.2014.41.033 To appear in: Geoscience Canada Received: April 2012 Accepted as revised: July 2013 First published on the web: January 2014 Articles ‘In Press’ are peer-reviewed, accepted articles that have undergone layout in journal
format, subsequent proof-editing by the author(s) and are ready for publication. The article in
this form is the version of record. The article will be removed from this page and paginated
once it is published as part of an issue. The date the article is first made available online as
either “Accepted” or “In Press” will be carried forward to the published article.
The article has been assigned a doi number that is finalized and citable. The doi number will be
consistent with the article through to issue publishing and will be activated in CrossRef when the
paper has been published in a Geoscience Canada issue.
GEOSCIENCE CANADA Volume 41 2014 1
SERIES
Remote Predictive Mapping 5.Using a Lidar Derived DEMto Test the Influence ofVariable Overburden Thickness and Bedrock onDrainage and Basin Morphology
Tim L. Webster1, John C. Gosse2,Ian Spooner3, and J. Brendan Murphy4
1Applied Geomatics Research GroupNova Scotia Community College50 Elliot Rd. Lawrencetown, NS, Canada,B0S 1M0Email, [email protected]
4Department of Earth SciencesSaint Francis Xavier UniversityAntigonish, Nova Scotia, B2G 2W5
SUMMARYA 4–m lidar digital elevation model(DEM) provides sufficient resolutionto examine the impact of variable tillcover on the incision history of multi-ple small (5 km2) catchments in easternCanada. The study site was selectedbecause it has homogeneous bedrockgeology that dips parallel to the landsurface, is tectonically stable, hasundergone common base levelchanges, and has a common ice history,with variable overburden thickness,from thin cover in the west to thickcover in the east. Basin morphometricswere compared for similar-size basinsthat have variable till cover thicknesses.Basins with thicker till cover are widerand show differences in hypsometriescompared to those where till cover isthin. Two basins representing endmembers of till thickness were meas-ured for stream discharge and waterchemistry. Thick till (> 1 m) on theeastern half of North Mountainretards infiltration sufficiently to pro-mote overland flow and accelerate inci-sion relative to areas with thinner till.Till thickness and continuity thereforeare expected to impede the achieve-ment of steadiness and may also delaystream power law relationships in larg-er catchments until till cover has beeneffectively eroded.
SOMMAIREUn modèle altimétrique numérique(MAN) par lidar 4 m offre une résolu-tion suffisante pour étudier l'impactdes divers dépôts de till sur l'histoire de
l'érosion linéaire de multiples petits (5km2) bassins versants dans l'Est duCanada. Le site d'étude a été choisiparce que sa géologie est homogène etque son pendage est parallèle à la sur-face du sol, qu’il est tectoniquementstable, qu’il a subi des changementssimilaires du niveau de base d’érosion,de même qu’ une histoire glaciaire sim-ilaire, avec une épaisseur de mort-ter-rain variable, d’une couverture mince àl'ouest jusqu'à une couverture épaisse àl'est. La morphométrie du bassin a étécomparée à celle de bassins de taillesemblable aux épaisseurs de till vari-ables. Les bassins aux couvertures detill plus épaisses sont plus larges etmontrent des différences hyp-sométriques comparé à ceux aux cou-vertures minces. Deux bassinsreprésentant les termes extrêmes de l'é-paisseur du till ont été mesurées quantau débit du courant et à la chimie del'eau. Les till épais (>1 m) sur lamoitié est du mont Nord retardent l'in-filtration, ce qui favorise l'écoulementen surface et accélèrent l’érosionlinéaire par rapport aux zones cou-vertes de couches de till plus minces.On s’attend donc à ce que l'épaisseurde la couche de till et sa continuitéagissent comme une entrave à la stabil-ité et puissent aussi retarder les effetsde la loi de puissance de l’écoulementdans les grands bassins récepteursjusqu'à ce que la couverture de till a étéeffectivement érodée.
INTRODUCTIONUnderstanding the relationshipsbetween stream incision and factorsrelated to fluvial erosion such as rock-uplift, climate, base level changes, andbedrock resistance to erosion (e.g. Seidl
et al. 1994; Gíslason et al. 1996; Stockand Montgomery 1999; Kirby andWhipple 2001; Stock et al. 2005) isimportant for the analysis of landscapeevolution (e.g. Pazzaglia 1993, 2003;Kooi and Beaumont 1996; Dietrich etal. 2003). The mechanics of river inci-sion have been studied by Whipple etal. (2000), Whipple and Tucker (2002)and related to rock strength by Sklarand Dietrich (2001). The availability ofhigh resolution (4 m) Light Detectionand Ranging (lidar) DEMs can facili-tate quantitative analysis of incisionand basin morphometrics at sufficient-ly small scales to allow the examinationof factors controlling stream evolution.Fluvial processes in glaciated terrainare complex because glaciers andstreams may sequentially occupy thesame valleys but each can uniquelycontribute to erosion, making the rela-tive influence of glacial and fluvialprocesses on valley size difficult to dis-tinguish. In addition to glacial process-es shaping the landscape glacial tilldeposits can influence the permeabilityof basin sediments and can affectdrainage characteristics, such as overland flow and infiltration of precipita-tion. Brocklehurst and Whipple (2002,2004) and Montgomery (2002) usemorphometric analysis including hyp-sometry and valley cross-sections todifferentiate large catchments affectedby alpine glacial processes from othersthat were affected only by fluvialprocesses. They concluded thatalthough glaciers widen and deepenvalleys, significant relief enhancementsare limited to large alpine glaciers.Studies applying the stream power lawoften use the contributing drainagearea as a surrogate parameter forstream discharge, which in addition tothe local channel slope, controls thestream’s ability to incise the underlyingbed (e.g. Stock and Montgomery 1999;Snyder et al. 2000):
E = KAmSn (Eq. 1)
Where E is the erosion rate, K repre-sents the bedrock erodibility factor, Ais the contributing drainage area, and Sis the channel slope. The exponents mand n are typically derived empirically.Few studies, however, examine thelocal hydrological effects of surfacematerials, such as glacial till cover, and
groundwater interaction on discharge(Tague and Grant 2004). At the scaleof basin areas of tens of square kilo-metres and larger, factors such as over-burden thickness and the fracture den-sity of bedrock can strongly influenceinfiltration rates and affect peak annualstream discharge.
Although the effect of DEMresolution on measuring differenthydrologic and geomorphic propertieshas been examined (e.g. Wolock andPrice 1994; Zhang and Montgomery1994; Gao 1997; Zang et al. 1999;Walker and Willgoose 1999), most ofthese studies have focused on the dif-ferent effects of grid cell size interpo-lated from similar source data fromphotogrammetry rather than advancesin data acquisition technologies such aslaser altimetry. In this study, the high-resolution of the lidar (Light Detectionand Ranging) DEM allows detailedanalysis of basin morphometrics toassess the local effects of variableoverburden thickness within a region.Lidar is a remote sensing techniqueused to derive precise elevation meas-urements of the earth’s surface (Ritchie1995; Flood and Gutelius 1997; Wehrand Lohr 1999). It has been used in alimited number of geoscience applica-tions, including the analysis of rivernetworks (Kraus and Pfeifer 1998;Gomes Pereira and Wicherson 1999;Stock et al. 2005), the generation ofriver floodplain cross-sections (Charl-ton et al. 2003), the investigation oflandslides (McKean and Roering 2004),and the mapping of tectonic faultscarps (Harding and Berghoff 2000;Haugerud et al. 2003), and bedrockcontacts (Webster et al. 2006a). Web-ster et al. (2006a) used lidar data alongwith field observations to revise thebedrock geology and map three indi-vidual volcanic flow units within theNorth Mountain Basalt which werethen used to categorize stream incisiondepths (Webster et al. 2006b).
In this study a high-resolution(ca. 4 m) laser altimetry (lidar) DEMwas used to examine metrics of simi-lar-size catchments that have beenmodified by glaciation. The FundyBasin area was selected for this studybecause (i) the catchments are devel-oped on three shallowly dipping vol-canic flow units of the Jurassic NorthMountain Basalt (NMB), each of
which have uniform resistance to ero-sion (Figure 1), (ii) the area is tectoni-cally inactive, (iii) the Bay of Fundyprovides a uniform base level for allstreams, (iv) there is a clear distinctionin till cover thickness over the easternand western portions of the study area,and (v) the age of deglaciation andsubsequent fluvial erosion is well docu-mented and uniform throughout thearea. The local effects of the variabletill cover on basin morphology and theinteraction of surface and groundwateron net discharge and stream powerwere evaluated. This study benefitedfrom previous work related to validat-ing the accuracy of the lidar DEM(Webster 2005), and individual points(Webster and Dias 2006), mapping thebasalt flow units (Webster et al. 2006a),and relating the stream incision depthto the basaltic flow units (Webster etal. 2006b). The objectives of this paperare to examine the morphology of sev-eral drainage basins derived from ahigh resolution lidar DEM to betterunderstand landscape evolution of asection of North Mountain of the Bayof Fundy area in Nova Scotia. Theeffects of variable overburden thick-ness, rock type and drainage character-istics are evaluated with respect tolandscape evolution. Drill core data ofthe NMB volcanic flow units wereexamined and fracture density meas-ured to test the influence of bedrockfractures on erosion and drainage with-in each drainage basin. In addition,stream discharge and water chemistrywere measured in two basins of similarsize to determine the contribution ofoverburden to stream discharge.
PHYSIOGRAPHY AND AGE OF THELANDSCAPEThe study area is situated along a 20-km section of North Mountain, main-land Nova Scotia, Canada, which com-prises the eastern shore of the Bay ofFundy, known for the world’s highestsemi-diurnal tides. The MesozoicFundy Basin is predominantly under-lain by Triassic sedimentary rocks (Blo-midon and Wolfville Formations), con-formably overlain by the JurassicNorth Mountain Basalt (NMB) to thenorth and unconformably overlain byPaleozoic rocks of the Meguma Ter-rane to the south (Fig. 1). The NMBdips gently to the northwest, forms the
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southeast limb of a regional syncline(Withjack et al. 1995), and is crosscutby north to northeast-trending faultsand fractures that exhibit dextral dis-placement (Olsen and Schlische 1990;Schlische and Ackermann 1995). TheNorth Mountain Basalt consists ofthree distinct flow units (Kontak,2002). The lower flow unit (LFU) con-sists of a thick (40–150 m) single, mas-sive, coarse-grained flow. The middleunit (MFU) consists of multiple thin,fine- to coarse-grained, highly amyg-daloidal flows (~3–15 m) with a cumu-lative thickness of 170–200 m. This is
overlain conformably by a massive,coarse- grained upper flow unit (UFU)of variable, but poorly constrained,thickness (Kontak 2000; Pe-Piper2000). The LFU is locally fracturedand exhibits well- developed columnarjointing. A conjugate joint pattern isvisible at the outcrop scale and on theshaded relief models of this flow (Fig.1). The MFU is locally fractured andeasily eroded when exposed at the sur-face, whereas the UFU has columnarjoints that have been sealed with sec-ondary minerals.
The maximum relief of the
study area is 265 m (from sea level tothe top of the North Mountain). TheNMB dips approximately 6o to thenorthwest (i.e., toward the Bay ofFundy) and on a regional scale, theland surface slopes at 3o to 5o in thesame direction. The region has a modi-fied continental climate strongly influ-enced by the adjacent Atlantic Ocean.Based on meteorological records fromEnvironment Canada the annual meanprecipitation is 1127 mm/yr, of whichan average of 276 mm occurs as snowand 910 mm as rain. The wettestmonths are September and Octoberwhen the average rainfall is 97mm/month and the average daily tem-perature is 6.8oC (Environment Cana-da 2005). The average daily tempera-ture drops below zero in the month ofDecember, reaches a minimum of -5.6oC for the month of January, andrises above zero in the month of April.The land cover on the North Moun-tain is influenced by varying thickness-es of glacial till. Farmland (pasturesand hayfields) and mixed forest domi-nate in the east where the till is thick-est, whereas the land in the west isdominated by mixed forest cover.There are more roads and anthro-pogenic influences in the east com-pared to the west where only onepaved road occurs along the coast. Thetopography of the coastline variesbetween gently sloping bedrock plat-forms and ca. 25 m cliffs that occur inembayments.
The region was affected byfluctuations in Late Wisconsinan icedynamics until ca. 12 ka (14C yr) (Steaand Mott 1998). The earliest ice flowswere eastward and southeastward froman Appalachian or Laurentide icesource ca. 75-40 ka (Stea et al. 1998).The Hartlen Till was deposited as aresult of the southeastward ice flowand typically consists of 40% gravel,40% sand and 20% silt and clay (Lewiset al. 1998). The second major ice-flowwas southward and southwestwardfrom the Escuminac Ice Centre in thePrince Edward Island (PEI) region(Escuminac ice flow phase 2, ca. 22–18ka; Stea et al. 1998). The resultingLawrencetown Till (Stea et al. 1998) isa reddish muddy unit that has higherclay content than the underlyingHartlen Till due to the incorporationof Carboniferous red bed sediment
Figure 1. (a) Upper right study area location map (black rectangle). (b) Bedrockgeology map (after Keppie 2000) of the Fundy Basin (heavy black rectangle) withlidar DEM location (heavy grey line). (c) Lidar DEM shaded relief model, azimuthangle 315, elevation angle 45; 5 times vertical exaggeration with the distribution ofthick till blanket (TB) and thin till veneer (TV) (after Stea and Kennedy 1989).
derived from Prince Edward Island,and typically consists of 20–30% grav-el, 30–40% sand, and 30–50% silt andclay (Lewis et al. 1998). Ice thenflowed northwestward and southwardfrom the Scotian Ice divide across theaxis of Nova Scotia at 18-15 ka (Steaet al. 1998). In a few localities, theLawrencetown Till is overlain by a thin(1–4 m) hybrid till related to this event,known as the Beaver River Till, whichgenerally consists of 50% gravel, 40%sand, and 10% silt and clay (Lewis etal. 1998). Locally ice flowed from theScotian Divide northwestward over theNMB into the Bay of Fundy. With thelate-glacial rise of relative sea level, iceflow into the Bay of Fundy increasedto merge with southwestward ice
streams from New Brunswick at ca.13–12.5 ka (Stea et al. 1998). The studyarea was ice free by ca. 12 ka (Grant1980).
The stratigraphy of the over-burden in the study area is not wellknown. However, where overburden ispresent it is dominated by theLawrencetown Till. In some areas thinpost-glacial marine deposits mantle thetill. Isolated kame and delta depositsalso occur in the region. Soil develop-ment is poor with ‘A horizon’ thick-nesses generally less than 30 cm. Steaand Kennedy (1998) describe the areasof thin < 1 m overburden as ‘scouredbedrock’ or a ‘till veneer’ (TV), whichoccur in the western catchments (Fig.1), and the areas of thick > 1 m over-
burden as ‘till blanket’ (TB), whichoccur in the eastern catchments (Fig.1). There is a transition zone betweenthese two end-members, where catch-ments have a till veneer in their head-waters and a till blanket in their outlets.
The drainages on the Fundyside of NMB have evenly-spacedmainstems (1.5 km), similar catchmentareas (ranging from 2 to 8 km2) anddendritic drainage patterns with streamdensities ranging from 0.9 to 2.9km/km2 (Table 1; Fig. 2). The basinshave streams which range in maximumstream order of 1 to 4 based on themethod proposed by Strahler (1952).The basins with thicker till cover havehigher order streams defined on theNova Scotia Topographic Database
4
Figure 2. Labeled catchment basins (black lines) calculated from the lidar DEM for North Mountain. The blue lines denote themain truck streams within the basins. The two red stars denote the location of stream discharge and water chemistry sensors inPeck and Sabeans basins.
Table 1. North Mountain basin metrics derived from lidar DEM in RivertoolsTM. Catchment type: TV – till veneer; TR – transi-tion zone; TB – till blanket. Drainage density* calculated from the stream network on the 1:10 000 scale topographic map.
Drainage Density Drainage Density* Source DensityCatchment Area Relief Stream from DEM streams from mapped streams from DEM streamsType (km2) (km) Order km/km2 km/km2 streams/km2
1:10 000-scale maps compared to thebasins with thin till cover. Thestreambeds are typically 80% bedrockand 20% boulder-covered, and lackaggregation fluvial deposits. Till ispresent in the streambeds of some ofthe basins in the till blanket area,attesting to the youthfulness of thesecatchments and to the inheritance ofsome low relief, pre-glacial topography.The long profiles of the streambedsare ungraded and have several knickzones.
METHODS
Lidar and DEM Analysis Details of the lidar data specificationsand height validation results for thisstudy are described in Webster (2005).Lidar data were acquired for the studyarea with an average ground pointspacing of 2–3 m in open areas and5–8 m in forested areas. Customizedautomated Arc/InfoTM GIS routinesfor the validation of the lidar pointdata are available in Webster and Dias(2006). The height validation resultsindicate that the original lidar groundpoints and the derived DEM are, onaverage, typically within 15 cm ofmeasured GPS heights and 95% of thedata are within 30 cm for open hardsurfaces (i.e. roads, parking lots). Thelidar ground points were used to con-struct a ‘bald earth’ DEM at a 4 m res-olution utilizing the ESRI suite ofArcGISTM v. 8 and 9 software. A com-bination of RivertoolsTM v. 3 and PCIGeomaticaTM v. 9 software was used toextract the morphometric parametersfrom the drainage basins (Fig. 2).
Catchment basins were calcu-lated for the main streams draininginto the Bay of Fundy from the lidarDEM based on outlet locations identi-fied on 1:10 000 scale topographicmaps using RivertoolsTM. The resultantbasin metrics are presented in Table 1.The standard D-8 algorithm (Jensonand Domingue 1988; Costa-Cabral andBurges 1994) was used to determinedown stream flow direction and sinks(depressions within the DEM treatedas errors by the algorithm) are filled inthe DEM to allow continuous downstream flow. At most resolutions, caremust be taken to consider that somelandscape metrics are fractal, such asrelief and slope (Anhert 1970; Van Der
Beek and Braun 1998; Zhang et al.1999). For this study the emphasis ison catchments with similar size so wehave not chosen a fixed-scale averagingmethod—this allows us to examine thestreams with maximum DEM resolu-tion. However, when dealing withDEMS at high resolution, other con-siderations must be made regardingfeatures such as sinks in the terrainmodel that are common in typicalDEMs derived from photogrammetry.Inspection of the drainage basinboundaries and stream longitudinalprofiles indicates that most catchmentshave sinks. Many of these sinks areadjacent to the raised elevations of aroadbed captured by the high resolu-tion of the lidar DEM. As a culvertcould not be represented on the DEM,a ‘notch’ was cut across the roadbedand assigned an elevation of the near-est downstream cell to improve theaccuracy of the flow direction algo-rithm and to prevent excessive erro-neous sink filling operations in derivingthe catchment basins and stream pro-files. This modification improved accu-racy of the flow direction algorithm,prevented excessive erroneous sink-fill-ing operations in deriving the catch-ment basins and stream profiles, andallowed the stream to ‘pass through theroadbed.’ The overall result is the gen-eration of a more accurate flow accu-mulation grid and basin boundary.
Eleven catchment basinsdraining the NMB into the Bay ofFundy were extracted from the DEMusing RivertoolsTM (Fig. 2). Table 1presents the morphometries of theextracted basins including drainagearea, relief, maximum stream orderfrom RivertoolsTM drainage densityfrom RivertoolsTM and drainage densitybased on the streams mapped on the1:10 000-scale Nova Scotia Topograph-ic Database map series. The drainagedensity of the basins is similar, regard-less of the stream order used in River-toolsTM. Drainage densities calculatedfrom the 1:10 000 topographic mapstream networks showed more variabil-ity (denoted as Drainage Density *from mapped streams in Table 1).
Morphometric AnalysisValley cross-sections, transverse to theoverall basin slope and extending fromthe lateral drainage divides, were
extracted for each of the basins inorder to evaluate the incision depthusing a method similar to thatdescribed in Montgomery (2002).Montgomery (2002) used the area ofthe cross-sections to calculate the inci-sion depth and cross-section areabetween the drainage divides. Valleybottoms in the cross-sections werealigned midway upstream of each basinin order to facilitate a comparison invalley shape between the two basinend-members, the scoured bedrockand till blanket basins (Fig. 3). Integrat-ed valley cross-sections were used tocompute the volume of materialremoved from each basin as describedin Mather et al. (2002). This approachassumes each cross-sectional area isrepresentative of the erosion betweencross-sections and is used to summa-rize the total volume of material erod-ed.
The elevations associated withthe drainage divides were used to con-struct a paleosurface of the NMB fol-lowing a similar method to thatdescribed by Brocklehurst and Whipple(2002) and Montgomery and López-Blanco (2003). The drainage dividesthat were calculated from the water-shed extraction process were overlaidwith the lidar DEM and elevationsextracted. These elevations areassumed to represent regions of mini-mal erosion and were used to interpo-late a surface that represents the paleo-terrain prior to erosion of the drainagebasins. The lidar DEM was then sub-tracted from this paleo-terrain surfaceto form an erosion map in order toquantify the incision depth and volumeof material removed by glacial-fluvialprocesses and the patterns of erosionfor each basin. The area associatedwith each erosion depth range was cal-culated for each basin. The NMB flowunits (Webster et al. 2006a, b) wereused to calculate the percentage ofeach unit per basin and the associatederosion depth per flow unit per basinand are presented in Table 2.
Bedrock Resistance to Erosion Sklar and Deitrich (2001) tested thestrength of various rock types to ero-sion and related it to river incision intobedrock. In this study, the resistance ofbasaltic flow units to erosion by abra-sion and plucking was tested. Litholog-
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ical resistance to erosion by abrasionwas tested in laboratory experimentson basalt flow unit samples using ashatterbox that consists of a cylindricalcontainer that holds a central disk andan outer ring. Samples were crushedand sieved to between 2 and 5 mm indiameter and placed in the shatterboxwhich was agitated for times rangingfrom 2–15 minutes for samples fromthe MFU to 20 to 40 minutes for thosefrom the UFU and LFU based on their
resistance to abrasion. The sampleswere weighed prior to agitation in theshatterbox, then sieved and weighedafter a set time of agitation to measurethe change. The results of the shatter-box experiments are presented in Table3.
Potential erosion by pluckingwas quantified by measuring the degreeof fracturing in the basalt. Lineamentsobserved on the shaded relief lidarDEM maps (e.g. Fig. 1) and aerial pho-
tos are large-scale fractures that do notappear to control erosion in thestreambed. Consequently drill cores ofthe MFU and LFU of the NMB wereused to quantify the fracture density ata smaller scale and the distribution ofvesicles and zeolite-bearing amygdules.Approximately 210 m of basalt wererecovered from drill hole GAV-77-3located 20 km east of the study area(Comeau 1978). The distinctionbetween individual flows was based onthe degree of oxidization of the flowand the amount of vesicles and amyg-dules of different sizes and characteris-tics (i.e., large or small, stratified orbubble pipes). Magnetic susceptibilityand rock quality designation (RQD), anengineering property that computes thepercentage of cumulative length ofcore segments longer than 10 cm overa 1 metre interval and the number offractures were measured for everymetre of core.
Surface and Groundwater InteractionThe effect of glacial till cover on sur-face water and groundwater interactionand stream discharge was evaluated bycomputing hydrographs and measuringwater chemistry parameters in two ofthe catchments with contrasting thick-nesses of till cover. These catchmentbasins, which were selected becausethey represent basin-type end-mem-bers, scoured bedrock till veneer (TV)and till blanket (TB), are of compara-ble size (Table 1). Thus, they can beused to test whether or not drainagearea is an appropriate surrogate meas-
6
Figure 3. Mid-basin cross-sections of catchments from west to east on NorthMountain with variable amounts of till cover. Cross-sections for the approximatemidpoint of each basin are labeled: 1 – Peck, 2 – Gaskill, 3– Sabeans, 4 – Phinneys,5 – Poole, and 6 – Starratt.
Table 2. Basalt flow unit percentage per catchment, average stream incision depths for each flow unit per catchment, overallaverage incision depth per catchment, and incision rate per catchment assuming a start time at 12 ka(thousands of years). TV –till veneer, TR transition between till veneer and till blanket, TB – till blanket. Incision rate km/Ma – km (kilometres) per Ma(millions of years).
Catchment % Drainage Incision % Drainage Incision % Drainage Incision Average Maximumarea LFU depth area MFU depth area UFU depth incision incision
ure for discharge in glaciated terrains.The thin TV cover is represented bythe Peck Brook catchment and thethick TB cover is represented by theSabeans Brook catchment (Fig. 2). Thestreams in each catchment were meas-ured to record hourly stream dischargeand water chemistry parameters fromApril to July 2004. Pressure transducerswere placed in or near culverts torecord stream stage near the outlets ofPeck Brook and Sabeans Brook catch-ments (locations shown by red stars inFig. 2). Manning’s equation was used tocalculate flow velocity and convertstream stage to discharge for the twochannels (e.g. Rose 2004). Multimeterswere also deployed near the outlet ofthese two basins to record water chem-istry parameters, temperature, pH, spe-cific conductance, dissolved oxygen,and turbidity. Meteorological stationswere deployed throughout theAnnapolis Valley and on North Moun-tain to measure meteorological eventswhich were correlated with the streamdata. The stream discharge, waterchemistry and meteorological condi-tions were integrated and streamhydrographs were constructed for thetwo basins. The hydrographs were nor-malized by basin drainage area asreported in Tague and Grant (2004).
RESULTS
Lidar and DEM Analysis The enhanced spatial resolution oflidar and the ability to penetrate the
vegetation canopy allow subtle topo-graphic features to be highlighted. Thecolour shaded relief (CSR) DEM wascompared to the established surficialgeology boundaries and glacial striationdirections (see Stea and Kennedy 1998)(Fig. 4a). The contrast in terrain rough-ness from west to east on NorthMountain is visible on the DEM(Figs.1, 4). The rough terrain in thewestern region of North Mountaincorrelates with glacially scouredbedrock, and the smoother terrain inthe eastern region correlates well withsurfaces covered by the LawrencetownTill (Stea and Kennedy 1998) (Fig. 4a,b). Two previously unidentified glaciallandforms are evident on the valleyfloor based on the CSR DEM (Fig. 4c).These landforms represent a set ofoval shaped drumlins trending 155o–355o and a set of streamlined land-forms trending 132o –312o. Based onfield verification, the set of oval land-forms in the western region, with along axis trending ca. 335o are com-posed of Lawrencetown Till draped bya thin layer of glacial marine lacustrineclay. The set of streamlined landformsin the eastern region, which has a longaxis trend ca. 312o is also visible onthe DEM maps (Fig. 4) and are com-posed of Lawrencetown Till. Streamson the eastern flank of the NorthMountain, where there is till cover,have flow directions of 312o, parallelto the streamlined landforms in thevalley (Fig. 4).
Morphometric AnalysisRepresentative cross-sections of theTV basins generally exhibit steep valleysides with flat bottom valley floors,whereas the till blanket (TB) basinscommonly have lower valley slopes andwider valley bottoms (Fig. 3; see Table1 for a complete list of basin namesand amount of overburden). The TVbasin valleys have broad gentle slopesin their headwaters and narrow steepvalleys closer to their outlets wherethey are incised into the more resistantLFU. The TB basins have lower slopesand wider valley bottoms along theirentire length (Fig. 3).
An erosion map highlights thedifferences in morphometry betweenthe scoured bedrock and till blanketbasins and the flow units of the NMB(Fig. 5). The TV basins have narrowincised valleys along their entire lengthand consist of only one main streamreach. The TB basins occur in broadervalleys with larger tributaries. Table 1shows the area and relief of each basinalong with the drainage density calcu-lated from the DEM and from the 1:10000-scale mapped streams denotedwith an * in the table. The drainagedensity calculated from the lidar DEMin RivertoolsTM is similar for all basins(Table 1). However, the drainage densi-ty is lowest in the basins with thin tillcover when the mapped streams areused (Drainage density *, Table 1).This is consistent with the higher max-imum stream order of mapped streams(orders 3–4) of the basins which have
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Table 3. Shatterbox experiment results for the North Mountain Basalt flow units. Upper flow unit (UFU), middle flow unit(MFU), lower flow unit (LFU), and weight percent (Wt %) of original sample for each sieve size in millimetres (mm). The timeeach sample was agitated in the shatterbox is reported in minutes (min).
Time (min) >2 mm >1mm >0.5mm >0.25mm >0.125mm >0.0623mm <0.0623mm Sample Rock_type
thicker till cover compared to those ofthin till (orders 1–2). The differencebetween the drainage density derivedfrom the DEM and that from themapped stream network is related tothe fact that artificial and ephemeralstreams are constructed from theDEM which are not represented on atopographic map as streams. Table 2highlights the areal percentage of thebasalt flow units for the three basin tillcover groups (TV, TR, and TB) andcomplements the spatial patternsobserved in Figure 5. The average inci-sion depth for each basaltic flow unitwithin each basin is also presentedalong with the average incision depthfor each basaltic flow unit based on allof the basins and for each entire basin(Table 2). The average incision depth isthe least for the TV basins (Peck 23.4m and Phinneys 28.5 m) in the westand increases eastward. The maximum
8
Figure 4. Colour shaded relief DEMmaps (a and b) compared with surficialgeologic boundaries and glacial stria-tions (black symbols from Stea andKennedy (1998) and white symbol stri-ations from this study). (a) Standardchromostereoscopic colour coding ofthe DEM to enhance overall terrainfeatures and relief, shading azimuthangle from 315o and zenith angle 45owith a five times vertical exaggeration.(b) Same colour ramp as above exceptit is scaled from 0–100 m, then repeat-ed in order to enhance the subtletopographic features at lower eleva-tions; shading azimuth angle changedto 225o to enhance northwest-trendingglacial landforms. (c) Same image as(b) with the landform trends highlight-ed by double headed arrows. The fieldof oval-shaped landforms trend155–335o, and the streamlined land-forms in the east trend 130–310o. Theterrain on both mountains east of theheavy line is smoother than the terrainto the west of the line. Surficial geolo-gy map labels: RB - raised beachdeposits; GSB– glacially scouredbedrock; LT – Lawrencetown Till; GM– glaciomarine lacustrine deposits; BT– Beaver River Till; KE – kame fieldsand esker systems; CD – colluvialdeposits.
average incision depths are associatedwith the basins within the transitionzone of till thickness (TR) for Gaskilland Poole basins at 42.3 m and 44.6 mrespectively. The average incisiondepths for the TB basins (Sabean 32.4m and Starratt 37.9 m) falls betweenthe incision depths of the till veneerand transition basins. The MFU lithol-ogy has the deepest incisions depthsfor all basins, regardless of till coverthickness, with an average of 45. 2 m.The LFU has an average incisionsdepth of 28.8 m for all of the basinsand is more resistant than the MFU.The UFU does not occur in all thebasins; however it shows the lowestaverage incision depth of 19.1 m.Maximum incision rates have been cal-culated for the catchments based onthe following assumptions: (1) fluvialincision began after deglaciation,reported to be at 12 ka ± 220 yrs byStea and Mott (1998), and (2) the tillcover was originally flat. The rates aremaxima because the tills probably hadsome relief and incision may havestarted prior to this time. The ratesrepresent preferential erosion of tillrelative to bedrock. These rates canvary up to 12.4% based on the accura-cy of the methods used to calculateincision depth and the date ofdeglaciation. The maximum incisionrate is highest in the catchments withinthe transition zone at 3.7 and 3.5km/Ma, followed by the catchments
covered by the thick till blanket at 3.2– 2.7 km/Ma and lowest for the catch-ments covered by a thin till veneer at2.4 – 2 km/Ma (Table 2).
Statistics associated with theerosion depth map (Fig. 5) include theareal extent and the volume of materialremoved for each catchment (Fig. 6).The erosion depth map is the differ-ence of the DEM with a paleo-surfaceconstructed from the elevations of thedrainage divides. The individual NMBflow units have been overlaid to indi-cate the differences in erosion relatedto the variable resistance to erosion ofthe flow units which is discussed inmore detail in the next section (Fig. 5).The TV basin end-members (Peck andPhinneys Brook catchments) show thelowest area and volume of erosionwith the TR and TB basins showingthe highest area and volume of erosion(Fig. 6). The most sediment removed isfrom the Starratt Brook catchment,which is covered by a thick till blanket(Figs. 5, 6). The volume of sedimentremoved for the rest of the catchmentsfollows a similar trend as the incisiondepths (Fig. 5), with the catchmentswithin the transition zone having themost sediment removed and the leastsediment removed in the thin till covercatchments (Fig. 6).
Bedrock Resistance to Erosion The bedrock resistance to erosioninfluences the incision depth as can be
seen in Fig. 5. The average incisiondepth is greatest for the Middle FlowUnit at 45.2 m, followed by the LowerFlow Unit at 28.8 m and the UpperFlow Unit at 19.1 m (Fig. 5; Table 2).There does not appear to be a patternrelating incision depth and the amountof overburden for the LFU (Table 2).However, the MFU has the greatesterosion depths in the basins with someglacial till cover. The maximum erosiondepth in the MFU occurs in the transi-tion catchments (TR) where till occursin the upper parts of the watershed,followed by the catchments covered bya till blanket (TB) and are lowest in thethin till veneer (TV) catchments (Fig. 5;Table 2). A similar pattern is observedfor the erosion of the UFU where ero-sion depth is greatest in the TR catch-ments, followed by the TB and lowestin the TV catchments. It appears thatthick till cover in the upper section ofthe watershed is important to possiblyprovide tools for erosion downstream.
As indicated by experimentalresults (Table 3), the MFU is muchmore susceptible to erosion by abra-sion than the UFU and LFU. Forexample, after 10 min. of abrasion,MFU sample BT17 had over 50% ofthe sample greater than 2 mm in diam-eter, whereas after 2 minutes of abra-sion MFU sample PC49 only had 32%of the sample greater than 2 mmdiameter after 2 min. (Table 3). Theresistance of the MFU is variable
GEOSCIENCE CANADA Volume 41 2014 9
Figure 5. North Mountain drainage basin boundaries (black lines and labels) and erosion depth map with basalt flow unitboundaries (white lines and labels: Lower Flow Unit – LFU, Middle Flow Unit – MFU, Upper Flow Unit - UFU). The westernbasins have maximum incision depths of approximately 40 m and the central and eastern basins have maximum incision depthsapproaching 80 m.
depending on the density of vesiclesand amygdules. Drill core analysisshows that the highly vesicular andamygdaloidal MFU has a higher rockquality designation (RQD) than theLFU, indicating a higher percentage ofrock segments longer than 10 cm (Fig.7). The UFU and LFU broke down atsimilar rates and are significantly moreresistant to abrasion than the MFU.The LFU has a greater number offractures per metre than the MFU anda lower RQD indicating fewer seg-ments greater than 10 cm in length permetre. The UFU does not occur in thedrill core, however field observationsindicate that secondary minerals havesealed fractures in this unit and thaterosion by plucking is less prevalent inthis unit than in the MFU or LFU. Theresults of the stream incision calcula-tions for each basin are consistent withthe shatterbox experiments whichshow the MFU is the least resistant toerosion followed by the ULF and LFU.The variation in stream incision depthsappears to be related to several charac-teristics of the bedrock lithologiesincluding the resistance to abrasion,the frequency and spacing of fractures,and the occurrence of bedding planesbetween flows (Fig. 7). Based on fieldobservations of plucking in the
streambed, the high fracture density ofthe LFU controls erosion for this unit(Fig. 7). Erosion of the MFU is con-trolled by fractures and bedding planesassociated with thinner flows and itssusceptibility to abrasion that corre-lates with the concentration of vesiclesand zeolite-filled amydgules asobserved in the drill core (Fig. 7).
Surface and Groundwater InteractionThe normalized hydrographs of PeckBrook (TV) and Sabeans Brook (TB)catchments indicate that discharge aftera rainfall event is much higher in theSabeans Brook catchment (e.g. 0.75m3/km2 sec on 4/23/2004) than in thePeck Brook catchment (e.g. 0.05m3/km2 sec on 4/23/2004) (Fig. 8a).The response time of the hydrographsbetween the two catchments is similar.However, the normalized discharge ofSabeans Brook is greater than PeckBrook and may be attributed to differ-ent rates of evapotranspirationbetween the catchments, because thePeck Brook catchment has more forestcover and less cleared agricultural landthan the Sabeans Brook catchment.The water in Sabeans Brook is general-ly more turbid after a rain than in PeckBrook. However, after a significant rain
event the specific conductance inSabeans Brook decreases, whereas itincreases in Peck Brook (Fig. 8b).Based on the hydrographs, SabeansBrook receives more overland flowthan Peck Brook after a rain. Thedominant hydrologic process of over-land flow for Sabeans Brook catch-ment and infiltration for Peck Brookcatchment is consistent with the waterchemistry data (Fig. 8b). The higherturbidity in Sabeans Brook comparedto Peck Brook is likely the result of tillmaterial washing into the stream. Thedecrease in specific conductance of thewater in Sabeans Brook after a rainevent is considered to represent dilu-tion, and the increase in Peck Brook isrepresentative of increased base flowof water that has had a longer resi-dence time in contact with the bedrock(Hem 1985; Winter et al. 1998). Thedominant process of overland flow inthe Sabeans Brook catchment com-pared to infiltration in the Peck Brookcatchment is attributed to the lowerpermeability of the Lawrencetown Tillthat covers the Sabeans Brook catch-ment area.
DISCUSSIONThe streamlined landforms in the val-ley floor and the alignment of theupper reaches of streams in the tillblanket (TB) catchments are likely tobe a result of the Scotian ice phase,which appears to have been the last iceadvance to affect this area (Stea andMott 1998). The streamlined land-forms and distribution of till suggeststhat an ice stream may have flowedfrom South Mountain across the valleyinto the Bay of Fundy (e.g. Stokes andClarke 2001). The larger catchmentsare associated with thicker till cover(Fig. 2; Table 1). The variation in sedi-ment volume removed in these catch-ments (Fig. 7) is consistent with thevariation in morphometric measure-ments (Figs. 3, 5). The maximummapped stream order is lowest in thebasins with thin till and highest in thetransition zone and basins with thickertill. This may be a combination of thetill being easier to erode and the influ-ence of the ice stream to form topo-graphic depressions that have beenused as preferential pathways for thestreams.
Others (Kirkbride and
10
Figure 6. Graph of area and volume of erosion for each basin along the NorthMountain Basalt. The basins have been grouped based on the degree of glacial tillcover: TV – thin till veneer, TR – transition zone with till in the headwaters, andTB – thick till blanket. The thicker light grey bars represent total area (left axis) ofmaterial removed per basin and the thinner dark grey bars represent the total vol-ume (right axis) of material removed per basin.
Matthews 1997; Li et al. 2001) haveexamined valley cross-sections to showdifferences in shape between glaciatedand non-glaciated valleys. In this study,the glacial processes appear to haveover-deepened and widened the catch-ments in the eastern region of thestudy area where the till blanket isthickest (Figs. 3, 5; Hallet et al. 1996).Differences in basin morphologyappear to be in part related to the vari-able till thickness and its influence onsurface and groundwater interaction,which in turn has a strong effect onstream power. The erosion depth mapshows the combined effect of variablebedrock lithology on stream incisionand the effect of glacial till cover onbasin morphology (Fig. 5). This indi-cates that the basins in the transitionzone and those with thick till cover aremore deeply incised than the scouredbedrock catchments and the till blanketcatchments have the best developeddrainage systems (Figs. 5, 6). The tillblanket catchments have more tributar-ies contributing to the main trunkstream than the till veneer catchments(Fig. 5; Table 1). These tributaries likelyform because overland flow is accentu-ated in the thick till blanket catchmentsdue to high clay contents and associat-ed low effective permeability of thesediment. This is consistent with theobserved stream discharge and waterchemistry (Fig. 8). The variable resist-ance to erosion of the basaltic flowsresults in the MFU unit having thegreatest incision depths (Table 2). Thisis attributed to the highly fractured andamygdaloidal character of this basalticflow as demonstrated in the abrasiontests and analysis of the fractures inthe drill core (Table 3; Fig. 7). Severalboulders of resistant LFU basalt occurin the stream beds of all basins. Webelieve these boulders act as tools inthe abrasion of the stream bed basalts.The occurrence of these tools in thetransition basins where till occurs inthe upper watershed and the glacial tillblanket basins promote erosion andthe MFU has the greatest erosiondepths for these catchments. The MFUalso has a density of fractures whichpromotes erosion by plucking and con-tributes to the deeper incision depthsfor this unit. As a result, the MFU hasundergone the deepest incision whichis attributed to the high susceptibility
GEOSCIENCE CANADA Volume 41 2014 11
Figure 7. Plots of drill core logs of drill hole GAV77–3 for the North MountainBasalt. The heavy grey vertical line marks the contact between the Middle FlowUnit (MFU) and Lower Flow Unit (LFU). (a) Magnetic susceptibility and the distri-bution of oxidized basalt, vesicles and amygdules interpreted to represent flowtops. The arrows denote individual flows within the MFU. The boundary betweenthe MFU and LFU occurs at depth 162 m. (b) Rock Quality Designator (RQD) %,which is the cumulative percentage of the number of pieces of core that are largerthan 10 cm over a distance of 1 m. (c) Number of fractures per metre length ofcore.
of this unit to break down throughabrasion and plucking.
Pazzaglia et al. (1998) notedthat stream power can control theshape of the stream longitudinal pro-file and that discharge is influenced bythe drainage area and infiltration char-acteristics of the basin. Many studieshave used drainage area as a surrogate
measure of stream discharge (Sklar andDietrich 1998; Snyder et al. 2000;Kirby and Whipple 2001; Whipple andTucker 2002; Mather et al. 2002; Fin-layson and Montgomery 2003; Stock etal. 2005) in estimating stream power(Equation 1). Montgomery (2002) useddrainage area as a normalizing parame-ter to compare metrics of valleys
affected by variable degrees of glacia-tion. Where basins are being com-pared, the use of drainage area to esti-mate discharge assumes that hydrologicprocesses affecting discharge in thosebasins are similar. Although thisassumption may be true at a regionalscale (1000 km2 and larger), the resultsof this study indicate that there can besignificant variations in hydrologicprocesses between basins within aregion (< 100 km2) and that drainagearea may not accurately correlate todischarge and consequently to streamincision. Monitoring the hydrographsand water chemistry of catchmentsfrom two basins with contrastingamounts of glacial till and land coverdemonstrate the strong influence thattill thickness and sedimentology has onrunoff and stream discharge. The dif-ferences in the hydrographs are a resultof the amount and rate of delivery ofsurface runoff for each catchment tothe stream. The till blanket catchmentspromote surface runoff (overlandflow) because of the low permeabilityof the till, whereas the scouredbedrock promotes infiltration anddelivers water more gradually to thestream through base flow. This is con-sistent with the findings of Tague andGrant (2004) who observed thathydrographs from basins in olderweathered volcanic rocks exhibitinglow intrinsic permeability were much‘flashier’ than those with younger,more permeable volcanic rocks in theCascades of Oregon.
The Lawrencetown Till has ahigh silt and clay content (30-50%) andlow permeability (< 10-6 cm/sec) mak-ing it suitable for liners in landfill sitesand other structures that require lowpermeable material (Lewis et al. 1998).Although there are no direct perme-ability measurements for the basaltflow units that we are aware of, theMFU and LFU are highly fracturedwhich would result in higher intrinsicpermeability’s (Fig. 7). Haan et al.(1994) reported a range of hydraulicconductivities for fractured basalt to bebetween 10-6 and 101 (m/day) and gla-cial till to be between 10-7 and 100
(m/day) and clay between 10-7 and 10-3
(m/day). Rose (2004) reportedhydraulic conductivities for fracturedbedrock to be between 10-8 and 10-4
(m/sec) and clay to be between 10-10
12
Figure 8. Hydrographs and water chemistry plots of representative basins fromthe till blanket (TB) area (Sabeans Brook catchment dark grey) and the scouredbedrock till veneer (TV) area (Peck Brook catchment light grey). (a) Hydrographsof Sabeans Brook and Peck Brook basins, discharge m3/s normalized by basindrainage area (km2) for the period of April 17 to May 31, 2004. Rainfall (mm) isplotted in reverse order on the right y-axis. (b) Specific conductance (microseimensper centimetre) and rainfall for Sabeans Brook and Peck Brook for the period ofSeptember 24 to October 28, 2004.
and 10-8 (m/sec). Pump tests from water wells
within the basalt aquifer yield an aver-age of 14.4 imperial gallons per minute(10 wells) and are prone to surfacecontamination where the till cover isthin (D. Fanning, pers. comm. 2004).These data further support our inter-pretation that the low permeable tillblanket promotes surface runoff andthe scoured fractured bedrock permitsinfiltration of precipitation. Withincreased overland flow, more waterenters the stream more quickly after arain and increases the overall dischargeand stream power, thus enhancing thestream’s ability to erode. This studydemonstrates that in glaciated terrains,factors such as a till thickness and sedi-mentology can affect the hydrologicproperties of a catchment and conse-quently the morphology of thedrainage basin. The high degree ofvariability in surface and groundwaterinteraction that can occur within a rela-tively small area can strongly influencethe nature of stream discharge. Thishas important implications for waterquality and quantity in glaciated ter-rains.
SUMMARYLandscapes within the study area aretypical of those found in northeasternNorth America that have been signifi-cantly eroded. The terrain is also repre-sentative of many areas in the worldthat have been eroded by fluvialprocess for millennia and during theQuaternary by glacial processes. Thisarea is an ideal site to examine factorscontrolling landscape evolution. Thecatchments are all generally the samesize, are underlain by similar lithologieswith relatively simple structures, andhave experienced the same changes inbase level. This facilitates an examina-tion of the effects of glacial till withina region on moderate-scale catchments.Studying the morphology of catch-ments at this scale allows examinationof factors affecting hydrologic andgeomorphic processes that can be usedto scale up to larger basins.
Airborne terrestrial lidar hasbeen a valuable tool in the analysis ofthese moderate-scale catchments andstream morphology within a forestedregion. In addition to basin morpholo-gy, the ability of the laser system to
generate high-resolution precise terrainheights in heavily vegetated terrains hasfacilitated the identification of geologiccontacts within the basalt units and theidentification of new landforms inter-preted to be related to late-stage icedynamics and fluctuations in sea-level.The catchments with the thickest tillcover have had the most sedimenteroded from them and differ in theirmorphology compared to the scouredbedrock catchments. In addition, thecatchments in the till blanket area havehigher drainage densities and deeperfluvial incision depths. We attributethese differences in catchment mor-phologies to reflect the influence ofglacial till on hydrologic and fluvialprocess and the possible addition ofmore tools in the streambed to pro-mote erosion by abrasion. The till hasa significant effect on the surface andgroundwater interaction by promotingoverland flow and retarding infiltration.The process of infiltration dominatesthe scoured bedrock catchments duringprecipitation where water is filteredthrough fractures into the groundwatertable and released as base flow into thestream as is evident from the hydro-graphs and water chemistry. The MFUand LFU are both highly fracturedwith an average of 7 and 10 fracturesper metre, respectively, allowing infil-tration in the thin till catchments. Inthe catchments with thick till cover, theincreased overland flow results in high-er discharge and stream power per unitarea compared to the thin till coveredcatchments. This has important impli-cations in glaciated terrains if thedrainage area is used as an approximatemeasure of discharge as is commonlythe practice.
The results of this study arewidely applicable to other glaciated ter-rains where vegetation cover obscuresthe topography. The ability of lidar topenetrate the vegetation canopy makesit an ideal tool for determining catch-ment morphologies in such areas.
ACKNOWLEDGEMENTSWe thank Ralph Stea and Dan Kontakof the Nova Scotia Department ofNatural Resources for field visits anddiscussions. We also thank Alex Mosh-er, Adam Csank, and Daniel Robertsfor assistance with the field work.Thomas Duffet of Dalhousie Universi-
ty kindly provided instructions on theuse of the shatterbox, and Cliff Stan-ley of Acadia University made availablethe drill core GAV-77. The lidar datawere supplied by the Applied Geomat-ics Research Group (AGRG) of theNova Scotia Community College(NSCC) and was funded by a CanadaFoundation for Innovation researchgrant from Industry Canada. TW alsothanks Bob Maher for his flexibilityallowing him time to work on thisresearch. Financial assistance wasprovided by the National Sciences andEngineering Research Council (viaJBM) and the NSCC to assist TW’sPhD study. JCG acknowledges sup-port from the Atlantic Canada Oppor-tunities Agency – Atlantic InnovationFund grant 1005052. We thank the tworeviewers of this manuscript who pro-vided important comments which haveimproved the manuscript.
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