Landform Analysis, Vol. 4: 99-107 (2003)

Sedimentological analysis of sandy-gravel accumulations,Serra da Estrela plateaux (Portugal)

Gon~alo Teles Vieira

University of LisbonCenlro de Es(udos GeograficosFaculdade de LetrasAlameda da Universidade1600-2/4 Lisboa. Portugal

Elzbieta Mycielska-Dowgiallo,Barbara Woronko

University of WarsawFaculty of Geography and Regional StudiesKrakowskie PrzedmieScie 3000-927 Warszawa, Poland

LAAb.wrucr: Sandy-gravel accumulations HI: frequent features on the plateaux of the Serra da Estrclaand other granitic mountains in Portugal. Their genesis has been attributed to both eolian and waterprocesses and a detailed sedimenlological study is needed 10 belief understand the mechanisms in­volved. The prcscm paper provides results obtained frum the heavy mineral analysis and optical andsc~nning electron microscopc study of samples from two sandy-gravel ~ccumulations from thc Serrada Estrela. The results indicatc that thc finer fr~ctions of the original sediment have been illuviatcd.A well-sortcd surface layer and poorly sortcd subsurface of the accumulation havc rcsulted from thisprocess. A very short dur~tion uf colian processes is evident from thc analysis. Although Wind seemsto have been the responsiblc for the morphology of thc accumulation, its action was not cnough toproduce significant abrasion in the surface of quartz grains.

Key 'Wlrds: wind ero,ion. water erosion. Scanning Electron Microscopy. Central Cordillera. Portugal

Introduction

The Serra da Estrcla is the highest mountainin Portugal (Fig. I) and reaches 1,993 m ASL inthc Torrc plateau. The central massif where thisstudy was conducted is granitic and mctasedi­ments crop out peripherally.

The mountain's plateaux form steps above1,400 m ASL (Fig. 2) and most of lhese wereglaciated during the Last Glacial Maximum(LGM). This episode greatly influenced the land­scape landforms of glacial erosion and accumu­lation are prominent. Bare-rock outcrops domi­nate in the glaciated area, while, beyond, a typ­ical tor and bomhardt morphology prevails.

Post-glacial weathering has disaggregated theexposed rock surfaces and has produced a thincover of granite gruss which is mobilised easilyby the present-day geomorphological dynamics.Despite being coarse sand and flne gravel, it isactively transported by wind and water process­es, and gives rise to small accumulations depos-

n'

ited against obstacles (Vieira, 1999). These arevery frequent in the plateaux of the Serra daEstrela (Fig. 3 and 4).

Similar deposits have been studied in theSerra do Geres, NW Portugal (Vieira, 1997,1999). The accumulations have slope angles of 10to 20" and are usually 20-50 cm long with a 100cm maximum and can be several metres wide.Three types of sandy-gravel accumulation havebeen identified: incipient accumulations; climb­ing tongues; and climbing tongues with blowout.The granulomctry of these deposits is of funda­mental importance. The superficial part of theaccumulations is a well-sorted layer, ca. I cmthick, with the mode in the -1.5 to -2.0 phi class.Below it, the material is finer and very poorlysorted. Comparison between samples from theSerra da Estrela and Serra do Geres, located about150 km apart, shows that the grain·size charac­teristics of the surface layer of the accumulations

99

Gonqalo Teles Vieira, Eltbieta Mycielska-DowgiaHo. Barbara Woronko

2 3km

101

Samples were taken from the two study sites(Fig. 2). The upper and lower sector and both thesurface and subsurface materials were sampled(Fig. 5).

Grain-size analysis of the coarse fraction wasperformed by sieving. Graphic representationsand measuring indexes were used for the charac­terisation of the samples (Folk & Ward, 1967).

Methodology

cm thick in the upper sector of the accumulationand 0.5 to I cm thick in the lower sector.

fig. 2. Location of the Fraga das Penas and Torre study sites in the Serra da Estrela (maximum extent of the LGMglaciers indicated by a dashed line, following Daveau, 1971)

Sedimenfological analysis of sandy-gravel accumulations, Serra da Estrela plateaux...

is available for transport and the accumulationsare generally small. The high elevation of theplateau and exposure to the western side of theSerra da Estrela make it an area where wind speedis high, an important fact that much influence inthe vegetation development.

The accumulation studied is a climbing tongue(Fig. 4) banked in Ihe herbaceous vegetation. Itis ca. 45 cm long and 100 cm wide. The slopegradicnt is close to 16° and the direction indicatesa wind blowing from SSW. The lower part of thetongue contacts directly with a flat surface withgranite cobbles and thin gruss, which correspondsto a wash sector. The superficial layer is ca. 0.5

Fraga das Penas

The sample site is located in the northern partof the Torre plateau at 1,920 m ASL. (Fig. 2). Itis a gently-sloping site covered by a herbaceousvegctation mat with bedrock outcrops in theconvex sectors. The lithology is the Estrela gran­ite, a medium-to coarse-grained muscovite gran­ite. In this site gruss appears in small vegetationfree patches and coarse sand accumulations arescarce. Owing to the vegetation cover little gross

of ca. 4°C (Vieira & Mora, 1998). Snow data arescarce and of poor quality. The median numberof days with snowfall at 1,400-1,600 m ASL is40 to 50 but there is a significant interannualvariability. This number increases with altitude(Andrade er al., 1992). Wind regimes are com~

plex and there are large variations depending onsite. Maximum speeds occur in Autumn andWinter, especially from November 10 March,when monthly speeds average 27 km/h. Thestronger winds are from Northwest to Southwestand the wind frequency is bimodal or even poly­modal, again depending on the site (Vieira &Mora, 1998). In general, the dominant directionsare West and Northwest.

The Fraga das Penas site is locatcd in thenorthern part ofthc Central Massifofthe Estrcla(Fig. 2) at the transition between the LOM gla­ciated area and the unglaeiated sector. It is asmooth, E-W-trending ridge, with tors, bombardtsand castle-koppjes. The bedrock is Seia granite,a coarse-grained porphyritic (wo-mica granitewith a relict Tertiary weathering mantle a fewccntimetres to a few decimctres thick. The sur­face comprises gross which is primarily mobil­ised by wash, rainsplash, wind and needle ice.Vegetation is dominated by ericaceous and dwarfjuniper shrubs but vegetation free patchcs areprominent, especially on the ridges where geo­morphological processes are more active. Thesandy-gravel accumulations arc mainly in windexposed areas on ridges, where sandy-gravelsource material is readily available.

The sediment sampled was deposited bywinds from WSW and is similar to the typeshown in Fig. 5. It is 33 cm long and severalmetres wide and has a slope gradient of ca. 10°.The surface layer is ca. 0.5 cm thick.

Torre

The Study sites

D<40Om

o 5""",

.>700m

D400·700m

. !} ,-~

U ,,---~

o 400km

2J) ~~~

z<{wU Serla da0 Estrela

U- ZZ<{ <{

Cl.

U)

37"N

100

fig. I. Location of the Serra da Estrela

are very similar from site to site. By contrast,subsurface material is very variable. .

Detailed geomorphological mapping, compar­ison with wind defomation of shrubs, and grain~

size analysis were the basis for the proposal ofa genetic model for the sandy-gravel accumula­tions (Vieira, 1997). The new results focus on thesignificance of their genetic processes and arebased on a substantially more detailed sedimen­tological analysis. involving optical and scanningelectron microscopy of the quartz grains and astudy of the heavy minerals.

The climate of the Serra da Estrela is Medi­terranean with dry and warm summers and a wetseason from October to May. Mean annual pre­cipitation is more than 2,000 mm in the higherparts of the mounlain and plateau (Daveau er al.,1977). Most of the plateaux have mean annualtemperatures below 7°C. The Torre is the coldestarea with an estimated mean annual temperature

Gon~alo Teles Vieira, Elibieta Mycielska-DowgiaHo, Barbara Woronko Sedimenlological analysis of sandy-gravel accumulations, Serra da Estrela plateaux...

Fir.;. 3. The slUdied sandy-gravel accumulation or lhc Fraga das Penlls SilC

/

\SSL

ssu-su \

SL\

/

99.99

95.00

90.00

1.00

10.00

'.00

99.00

99.90

Grain-size (Phi)

Fig. 7. Grain-size distribution in the TOrTC sandy-gravel accu­mulationSU - 11Irfa« l.yer upper .C'Ctor; SSU - 11Ibsllrface m.len.1 lIppersec:ur, SL - surface byer lo1r;er 5Cl;tor; SSL - subsurl"_ material lower

~"

0.01q ~ ~ ~ oq ~ q ~ q ~ q ~ q ~

~ ~ ~ 9 0 - - N N ~ ~ • ~

0.10

reflect the mineral composition of the bedrock.Dominating are biotite and opaque minerals(Table I). In the lower sector a distinct impov­erishment in lamellar minerals of the mica group(biotite, chlorite, glauconite, muscovite) is ob-

~

~ 70.00e.-~ 50.00

~ 30.00

I

/

su - surface upper lCC101; ssu _11Ib$lIrface upper Sec1or; SL - surraee lo...er seclor; SSL­sub~urrKe 10"..er $«:101

Table I. Content of heavy minerals in the studied samples (grain-size 0 1-02 mm)

"'/VIGA DAS I'£NAS TORR£

S.mpln 'U "u ,e m 'u >su 'e >seAmphibole 0.2 2.4 3.2 1.9 4.3

Biotite 81.3 74.7 71.2 68.0 4.8 7.8 6.5 19.0

Chlorilc 0.4 0.1 - 0.5 - 0.9 1.1

Zircon 0.1 - - - -Kyanitc 0.1 - - - - - -EpidolC - - 1.2 0.1 0.8 OA - 1.1

Glauconite - 0.9 - -Muscovite 2.3 1.0 - 1.3 76.3 79.6 77.5 61.6Piro)(cne - - - - 0.2

Rutile 0.8 - 1.0 -Sphene - - - - 0.8

Staurolile 0.8 0.6 0.8 OA 0.5

Tourmaline 0.1 3.8 2.0 4.7 4.3

Carbonate - - 4.0 - - 0.5

Opaque 15.2 24.0 27.0 29.8 5.5 4.6 8A 7.6

Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

\

su /

SL\

SSLI

--ssu

99.99

10.00

'.00

1.00

99.00

99.90

95.00

90.00

The beavy minerals of theFraga das Penas accumulation

Heavy mineral analysis

0.01

Grain-size (Phi)

FIg. 6. Grain-sizc distribution in thc Fraga das Pcnas sandy­gravel accumulationSU - surf.ce t.yn uppn SC'Clor; SSU _ lub.urf.ce m.ter••l uppersec:lor; SL -surfKe I.yer Io"'CT J«tor: SSL - subsurfKe mllml1lov>'CT~,~

0.10

The surface samples from the Torre site havesimilar grain-size distribution curves to those atthe Fraga das Penas (Fig. 7). The upper sector hasa graphic mean of -1.3 phi, it is moderately sort­ed, very positively skewed and very Jeptokurtic.In the lower sector the sedimentis coarser (graphic mean -1.7phi), well· sorted, very positivelyskewed and very leptokurtic. Thesubsurface material is differentfrom that at the Fraga das Penas.lltis difference is very common inthe coarse sand accumulationsstudied in the Serra da Estrela andSerra do Gcres (Vieira, 1999). Inthe upper sector, the graphic meanis 0.1 phi and in the lower sectoris 0.0 phi. As in the subsurfacesamples from the Fraga das Penassite, the sediment in the lowersector of the accumulation isslightly more mesokurtic than inthe upper sector. There is an in­crease in sediment coarser Ihan ­2 phi and also finer than 3.5 phi.

~ 50.00

~ 30.00

~

~ 70.00e.-

Grain-size analysis

Results

For the detennination of round­ing and frosling of quartz grainsurface under the light microscope,groups of 100 grains wcre sclectedfrom Ihe size fractions 0.7-1.0 mm(referred onward as the fine frac~

tion) and 1.0-1.4 mm (coarse frac­tion). At the Fraga das Penas sitegrain sizes over lA mm were alsoslUdied. The method of Cailleux(1942, 1961) modified by Mycielska­-Dowgialto & Woronko (1998) wasfollowed. Both surface and subsur­face grains of the upper sectors andsubsurface grains of the lower sec­tors of the accumulations wcre srud­ied. At the Torre site, the surfacelayer of the lower sector was alsoanalysed (Table 2).

For thc analysis of the micro­morphology of the surface of qllartzgrains in the scanning electron mi­croscope (SEM) groups of 6 typicalgrains were selected from each sam­plc using the light microscope. Thesurface layer of the upper sector andsubsurface layer of the lower sectorof the accumulations were studied.From the Fraga das Pcnas site grainswere chosen from the I.O-IA mmclass and from the Torre site theclass 0.7-1.0 mm was also studied.

The accumulations srudied show the typicalgrain-size characteristics of the sandy·gravel accu­mulations described by Vieira (1999) (Fig. 6).

In lhe Fraga das Penas accumulation the sur­face layer in the upper sector has a graphic meanof -1.3 phi and is moderately sorted, positivelyskewed and leplokurtic. In the lower sector itbecomes coarser (graphic mean -1.7 phi). wellsorted. very positively skewed and mesokurtic.The subsurface material is very distinct. In theupper sector, the graphic mean is -0.4 phi and itis very poorly sorted, very positively skewed andleptokurtic. In the lower part, Ihe graphic meanis -0.1 phi and the material is poorly sorted, verypositively skewed and mesokurtic. The subsur­face grain-size distribution is very similar. Thedifrerences in kurtosis appear 10 be ex.plained bythe sediment distribution in the ex.lreme classes.

Fig. S. I'osition of the samplcs m Ihe sandy-gravel accumula­lion

The heavy mineral distribution within thedeposit can be significant for the reconstructionof the morphodynamic processes. This was deter­mined for the 0.1-0.2 mm fraction.

Fig. 4. The: sludied sandy-gravcl accumulation of thc Torn: site

102 103

Gon~a/o Teles Vieira, Eltbieta Mycielska·DowgiaHo, Barbara Woronko

served. whereas, in the upper sector, there is anenrichment in these mincrals. In the surfacelayer, the tolal content of mica group mineralsamounts to 71.2% in the lower sector and to84.0% in lhe upper sector. A similar situation isobserved in the subsurface material (69.8% inthe lower seetor and 75.8% in the upper).

In the lowcr sector of the accumulation apronounced enrichment in opaque minerals maybe observed in comparison with the upper sector.This applies to both the surface and subsurfacematcrials (Table I).

The accumulation from the Torre site showsa richer heavy mineral spectrum, but, neverthe­less, these are still strictly related to the bedrockmineral composition (Table I). Muscovite dom­inates but there is a considerable content of bi­otite. Amphiboles occur in the group of mineralswhich are little·resistanl to weathering and mc·chanical abrasion. Considering the spatial distri­bution of the total content of mincrals of higherspecific gravity (opaque minerals plus tourma­line), a distinct enrichment is notable in the low·er sector of the accumulation in both the surfaceand subsurface materials. The enrichment of min­erals of lamellar structure of the mica group isvisible in the upper sector of the subsurfacematerial. Ln the surface layer, the distribution isthe converse of this and the total content of themica group minerals is slightly higher in thelower sector of the deposit.

Optical microscopy

prevail (46.(}-61.9%). The content of weatheredgrains in the fine fraction (0.71-1.0 mm) is muchlower (18%). Ln both size fractions the share ofcracked grains (EM/C) is much lower than in thesamples from Fraga das Penas. This share is from12.7 to 22.5%, whereas that of angular entirelyfresh quartz grains (NU) is considerably higher(17.2-36.7% in the coarse fraction and up to 66%in the fine).

Scanning Electron Microscopy

SEM analysis of the Fraga das Penas samplesshowed that none of the examined grains exhib­ited effects of eolian processes. especially onedges and corners, which are particularly sus­ceptible to eolian abrasion (Fig. 8). On the oth­er hand, on the majority of the examined grainsurfaces the effects of intensive physical (Fig.8-A and E) and chemical weathering (Fig. 8-8,C, D and F) arc pronounced. The chemical etch­ing becomes apparent due to the presence of ori­ented structures (Figs. 8-8 and F) and crystal­lisation of secondary minerals. Records ofweathering are visible in all depressions and oncracked surfaces.

As with the observations in the lighl micro­scope, the SEM samples from the Torre site re­vealed an almost complete lack of traces of eo­Han mechanical abrasion on edges and corners(Fig. 9). On the other hand, as with the Fraga dasPenas samples the majority of grains bear a his­tory of chemical and physical weathering.

The Fraga das Penas samplesshow that irrespective of grain sizeand sample sector, cracked, poorlyrounded grains with partial or totalfrosting of the surface are dominant(50-72.7%) (Table 2). Fwsted,poorly rounded grains withoutcracks (EM) comprise the secondmost important group (21.8-46.7%).They are charactcrised by a lowrounding degree (0.3-0.5 after thescale of Krumbein, 1941). Angularand completely non-abraded grains(NU) comprise 2 to 5% of the sam­ple. Their share is higher in the fin­er fraction (0.7-1.0 mm - 12.0%).

With respect to the Torre site.the counts showed that, irrespectiveof localisation within the deposit,among coarse fraction quartz grains,those with weak rounding and par­tial or total surface frosting (EM)

104

Table 2. Rounding and frosting analysis of quartz grains (%) using the cri~

teria ofCaillcux (1942) modified by Mycic:lska-Dowgiallo & Woronko (1998)

L""1Uo1 .." ttMIC

S....pl... NU PMr ra••dlal P_r .......1 P..r .....di.l

ra'irdy fru~ ••d lal.1 ud IM.I ar aod 101.1 er

r,..) 1I°1 parli.1 rronlol perll.1 r~li.

..1111 ~ru'"

'" SU (>1.4 mm) 5.0 1.0 21.8 72.7~

SSU (> 1.4 mm) 3.' 0.0 27.4 68.8'""-'" SSL (> lA mm) 4.0 0.0 44.0 52.0Cl SU (1.0-1.4 mm) 3.3 0.0 46.7 50.0

Cl SSL (1.0-1.4 mm) 2.0 0.0 45.1 52.9

2 SSU (0.71-1.0 mm) 12.0 0.0 26.0 62.0

SU (t .00t.4 mm) 36.7 4.7 46.0 12.7

'"SSU (1.0-1.4 mm) 32.7 0.0 54.0 13.3

'" SL (1.0-1.4 mm) 58.•'" 18.5 0.0 22.5~ SSL (1.0-1.4 mm) t7.2 0.0 61.9 20.8

SU (0.71-1.0 mm) 66.0 0.0 t8.0 16.0

su JlIrr.c~ UPIlC" sector. SSU - subsurface upper seclor, SL - surf.c~ lo"'~r "CIOI. SSL ­subsulr.c~ IO"'~r 5«10'

Fi~. 8. Scanning Eleclron Microscope photographs of quanz grains from the Fragll das I'enas sandy-gra\e! accumulation~ - ~on-lllraokd quanL gom "1\10 ,luIrp edg~s and COfnt'n,. IJ - 'lllaru gnllD .1tC'l"ed by thermal "'-~.Ihcnngprocess Ont1lled ~truc'u,es.R' 'lJ1bl~;

1'lI1!1l'ICn1 of quartz Ion,," ~urfltCc .h~rc-d b)' ch~.."nl ,,·ulh.,.,ng WIth oncn1l,d JITVCfUrel; D quartz gn,,' .h"!l'd b,' chemICal "Ulbrnll th:~:,~rK\;. E fr:agmcnl of qu'UU gn,," "Itb >HlOUS frC'$h brnbg~, du~ 10 ph)slCal "eatbennl;: f' _ ,,"coled )truelur~; \'slblc OD Ibr 'luM17f',;10

Sedimentological analysis of sandy-gravel accumulations, Serra da Estrela plateaux ...

Discussion

Crain size

The surface layers of the two accumulationsshow similar grain-size characteristics. Tbey fonna lag surface of well-sorted sandy-gravel. Thecoarsening of the lag in the lower sector lndicatesan increase in wash and probably an eolian sort­ing (coarser grains are more difficult to moveupwards in the accumulation slope). The subsur­face material is poorly sorted and grain-size dif­ferences occur between the deposits. These dif­ferences were found in accumulations studied atother mountain sites and were interpreted asrelating to the different parent materials and dif­ferences in the down wash of fines (Vieira, 1997).

In the case of the Fraga das Penas deposit, thehigher content of opaque minerals in the subsur­face material and lower sector is most probablythe effect of washing-out by the surface waterflow. At the Torre site, the same effect is present,but the differences are only significant in respectof the higher content of opaque minerals andtourmaline (specific gravity >3.0 g/cmJ

) in thelower sector.

At the Torre site, the slightly higher contentof the amphibole group in the subsurface mate­rial seems to indicate a deeper weathering with­in the modern surface.

To recapitulate, the distribution of heavyminerals indicates that two processes were in­volved in the formation of the sandy-gravel ac­cumulations: wind activity and surface waterflow.

Fig. 9. S<:anning Ele<:lron Mi<:ros<:ope photographs of quam: grains frOIll the Tone ~andy-gravel a<:culllulationA angular 4uart" grain; B - fragmcnt of the edge of quartz gra;o pre,eOlcd on A; C 4uaru ~,'r"io with minor change~ on the edge due to aealianahra.ion: 0 _ fragment of quartz grain with uricnlcd ,\ruclUrcs and Ihc cdge with .",me changes due tu aeolian abra,iun; E quart/. grato with thesurfacc significantly ahcred by chemIcal weathNtog; I' - fragmenl uf 4"arl" HTalll significanlly altcred by chemical weathenng

Heavy minerals

The analysis of the heavy mineral spectrumproduced interesting results. The high proportionof opaque minerals at Fraga das Penas may in­dicate that the Tertiary regolith is the source forthe deposit studied. At the Torre site, the presenceof amphiboles and a markedly lower content ofopaqoe minerals suggests a lower contributionfrom the Tertiary regolith. This is explained bythe location of the Torre site in the glaciated areawhere the Tertiary weathering mantle is absent.Data thus indicate a local origin for the sedimentforming the accumulations.

The differences in the proportions of micagroup minerals between the accumulation's lowerand upper sectors at Fraga das Penas apparentlyreflect their sensitivity to eolian selection process­es, in respect that this group is the first to beremoved from the deposit (Mycielska-DowgiaHo,1993). From this it is concluded that the differenc­es result from their removal by wind action fromthe lower sector of the slope and accumulation inthe upper sector, in front of a plant obstacle. Atthe Torre site, the participation of the eolian seg­regation process may be inferred only from themineral composition of the subsurface material.

The opaque minerals comprise a group of highspecific gravity (usually >4.6 g/cm\ This groupcontains various iron oxides and sulphides as wellas other minerals, which, in the weathering pro­cess, became coated with iron hydroxides andoxides. They are subject to a process converse tothat of the mica group and their segregation.owing to their high specific gravity is particular*ly effective in the aqueous environment or in thepresence of washing-out with water (Myeielska­Dowgiallo, 1995).

14 - Landfonn

Optical and scanning electron microscopy

In the studied samples, the frosting type vis­ible on most of the very poorly rounded grainsalready at low magnifications, indicates the ac­tion of intensive chemical and physical weather­ing in the formation of the surface relief. Oncracked surfaces, traces of initial frosting due toweathering are visible.

Similarly to the results of the heavy mineralanalysis, rounding and frosting of quartz grainsin Fraga das Penas indicates that these chieflyoriginate from the Tertiary weathcring mantle.This weathered material was probably subse­quently transfonned by frost shattering (a highproportion of broken grains). The rounding andfrosting of quartz grains at the Torre site indicatea stronger impact of physical weathering with ahigher proportion of entirely fresh grains. Theproportion of grains transformed by chemicalweathering (EM) (probably during the Tertiary),is similar at both sites.

Very few light traces of abrasive action ofeolian processes on edges and convex surfaceswere observed in the Torre samples and none inthose from Fraga das Penas. This suggests a shortduration of eolian processes within the transporthistory in both sites, but particularly at the Fragadas Penas. This conclusion is also supported bythe low degree of rounding of feldspar grains.The proportion of these minerals is significantdespite their low resistance to mechanical abra·sion as compared to quartz.

The SEM observations support the resultsobtained with the light microscope. The grainsurface analysis proved a very short duration ofmodern eolian proccsses; this is evident from the

105

Gon9alo Teles Vieira, Eftbieta Mycielska-Dowgiallo, Barbara Woronko Sedimentological analysis of sandy-gravel accumulations, Serra da Estrela plateaux ...

lack of a characteristic microrelief on quartz grainsurfaces. On the other hand, the results of bothchemical and physical weathering processes areclear, the lalter being responsible for both thehigh share of cracked grains and the grain sur­face frosting.

Conclusions

Hcavy mineral analysis and quartz grain sur­face texture studies using the optical and scanningelcctron microscopes provide new insights intothe characteristics and genesis of the sandy·gravelaccumulations at high elevations iD Portugal. Theresults support the genetic modcl by Vieira (1997)and emphasise the significance of wash process­es in the post-depositional evolution of the accu­mulations.

The analysis of the surface morphology oftbequartz grains indicates intense chemical andphysical weathering but almost no traccs of eo­lian abrasion. These observations suggest that thcaction of the eolian processes in the genesis ofthe deposits was short, i.e. not longer than 100­200 years (Mycielska-Dowgiallo, 1993; Myciel­ska.Oowgiallo el al., 1998). As follows fromearlier investigations (Mycielska-Dowgiallo,1993: Ovchinnikov, 1998), thc first indication ofcolian processes is the dune form, and only af·tcrwards arc the textural features of the sedimentsmanifested.

Plainly, the evolution of the accumulationswas controlled by a complex interaction of eolianprocesses and wash during rainfall or snowmelt.The first phase was eolian mobilisation by highwinds to move materials finer than fine gravelsand to deposit them in gently-sloping depositsabout 50-150 cm wide, 20-50 cm long and 5-10cm high. After deposition wash processes dom­inated thereby leaving a surface lag layer withsome of the fines downwashed into the deposit.Thus, the deposit is poorly-sorted at depth incontrast to the well-sorted character of the sur·face layer.

Acknowledgements

Professor Antonio de Brum Ferreira isthanked for the comments on the manuscript andsuggestions on field research. Professor PeterBirkeland and Dr. Peter Walsh are thanked for thecomments and suggestions, which very muchcontributed to enrich and clarify the article. TheNatural Park of the Serra da Estrela (P.N.S.E.)

106

gave logistical support during fieldwork. TheUniversities of Lisbon and Warsaw have fundedthis research.

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