Geological and pedogenetical formation and their relation to archaeological remains of the...

1Geological and pedogenetical formation and their relation toarchaeological remains of the

Crustumerium settlement, Lazio, Italy [M. den Haan, 2013]

Geological and pedogenetical formation and theirrelation to archaeological remains of theCrustumerium settlement, Lazio, Italy.

Michael den Haan

University of Amsterdam


Crustumerium settlement, Lazio, Italy [M. den Haan, 2013]20122013

contents

Problem statementGoals 3

Introduction into the geology 4

Geological units within Crustumerium 6

General introduction into Mediterranean soil formation 8

Soil formation within Crustumerium 10

Soil depth and sediment transport along transects 13

Description of several anomalies 17

Morphology of the Tiber valley 18

Conclusions 19

References 21



Problem statement

Archaeologists at the Groningen Institute of Archaeology (GIA), conducting a long-term landscape archaeological project in the prehistoric settlement of Crustumerium in Rome, have entered a new research phase for which a more detailed understanding of the physical environment and its effect on the archaeological record are required. As part of the PhD research of Jorn Seubersand The People and the State project initiated and directed by Prof. Attema and Dr. Nijboer, some preliminary studies were already conducted in 2010 (Eastern Atlas), after which it was concluded that a geopedological survey was needed totest approaches, and a deeper insight into the geology and soils of the area.

Goals

- teaching archaeologists to recognise and describe typical soil profiles for the study area, and any deviations thereof. A distinction must be made between ‘original, natural’ soil profiles and ones that have been altered by man, and between information that can be derived from boreholes and from sections;- achieve a greater understanding of how slope processes (erosion, colluviation) and terracing (cutting and filling of banks, plough erosion) are affecting the post-depositional preservation and identification of archaeological remains.- Correlate findings of previous geophysical studies with corings and pedological formation.

I will first introduce the geology and the pedogenetic processes and phenomena within the Crustumerium settlement, before describing the field research conducted in July 2012 and its consequences for the Crustumerium project futurework.



Introduction into the geology

The geological basis of the fieldwork area is characterized by a Pliocene (5.3 – 2.5 Mya) marine claystone bedrock. This marine claystone bedrock is hundreds of meters thick and has a very low permeability (Corazza et al., 2006). The Limestone is overlain by sandstone, claystone and conglomerates, formed under continental conditions during the lower to middle Pleistocene (2.5 – 0.5 Mya). These are in their turn overlain by middle Pleistocene (548 – 410 kya) volcanic deposits. The latter consist of thick resistant effusive rocks (lava) and pyroclastic material from, respectively north the Monti Sabatini and south the Alban Hills, volcanic complexes. The incising Tiber river cut down into these rocks, forming high, steep-walled plateaus on which defensible settlements like Crustumerium were built, and during the Holocene filled its valley with alluvial deposits (figure 1).

The lithified pyroclastic deposits are known by many different names. Pyroclastic material is the fragmented rock ejected by a volcanic explosion, hence the name pyroclastic (broken by fire). The material may be thrown out as molten lava, which cools into volcanic bombs (>64 mm), lapilli (2-64 mm) or ash(<2 mm). Scoria and pumice are formed when the hardened top of the lava is blown out by the eruption of the volcano. Both scoria and pumice have a vesicular structure, but pumice is more spongy giving it the property to float on water.The differentiation in names of effusive rocks is based upon the content of volcanic glass and pores. Ignimbrite consists of glass-rich pyroclastic flow deposits (ash) that were deposited while the glass was still liquid and lithified by cooling still after deposition. The ash is composed of glass shards and crystal fragments. Compacted, lithified effusive rocks composed of pyroclastic material that after deposition hardened due to zeolitisation (weathering with the formation of new minerals – zeolites) are called ‘tuff’. If not lithified the pyroclastic deposits are called ‘ash’.

There are several stages of eruptions that have resulted in the formation of distinctive volcanic rocks. Figure 2 shows a detail 2008 Geological map of Rome(Funiciello et al., 2008) for the Crustumerium fieldwork area. In the followingchapter the geological units present within Crustumerium are described in more detail. General characteristics and estimated date of formation are presented. The geological units were dated by Karner et al. (2001) by using the Ar40/Ar39 ratios of sanidine and/or leucite crystals.



Figure1. The geological and morphological history of Crustumerium. 1. Forming conditions of marine clay deposits. 2-3. First uplift in early Pleistocene, followed by deltaic deposits of paleo-rivers. 4-5. Second uplift of Monte Mario horst in middle Pleistocene followed by the formation of the ignimbrite plateaus of the Sabatini volcanic district and Alban Hills volcanic district. 6-7. During the upper Pleistocene until now the volcanic rocks were incised and characterized byfluvial deposits of the Tiber river and its tributaries. (After Funiciello et al., 2008)



Geological units within Crustumerium

VSN2a – Villa Senni Formation, Occhio di Pesce (Fisheye) (circa 357 kya) not visible on 1:50.000 geological map

This unit of ignimbrite belongs to the upper part of the Pozzolanelle member and is characterized by leucite crystals with a diameter of up to 2cm and more than 30% in volume. The large leucite crystals show as characteristic white dots, giving this ignimbrite the name ‘Occhio di Pesce’ or ‘Fish-eye’. This abundance of crystals goes together with a general percentage of >15% of lithic coarse crystalline fragments with a diameter of several decimeters. The ignimbrite only outcrops in the southeastern quadrant and presumably originatesfrom the Alban Hills caldera.

NCF – Unita di Nuova California (part of the Sintema Quartaccio (QTA) (~340kya) not visible on 1:50.000 geological map

This geological unit is the fill of the dissected surface caused by the QTA. The lowest parts of this unconformity are ascribed to marine isotope stage10 (~340 kya). The low energy and lacustrine gravel deposits consist of low quality flint. Impurities in the flint limit the usability for paleolithic tools. LTT – Tufi Stratificati Varicolori di la Storta (circa 410 kya)

The predominant geological unit in the fieldwork area is the Tufi Stratificati Varicolori di la Storta (LTT). Karner (2001) dated this Ignimbriteat around 410 kya. The LTT is composed of a complex sequence with high amounts of yellowish white pumice and gray scoria. The pumice contains abundant sanidine (potassium feldspar), leucite (potassium aluminum silicate) and pyroxene. The maximum thickness of the geological unit LTT was measured in the cuts made along the railway line Rome-Viterbo, in which the measured maximum thickness was 10 m (Funiciello et al., 2008).Two facies have been recognized separated by a layer of white pumice, in the older layer Paleosols have formed. These Paleosols have also been identified inoutcrops of the Crustumerium site. SKF – Tufi Stratificati Varicolori di Sacrofano (circa 488 - 514 kya)

The Tufi Stratificati Varicolori di Sacrofano (SKF) has outcrops in the northeastern and southwestern borders of the Crustumerium site. The SKF consists of non-lithified stratified ash deposits with a maximum thickness of 180 cm. The sequence is composed of non-lithified layered yellow pyroclastic deposits with a fine grained matrix. All deposits are described by fallout fromnorth to south and are interpreted as eruptions from the Sabatino.volcano centers.

GPS – Grottarossa Pyroclastic Sequence (514 kya) not visible on 1:50.000 geological map


Crustumerium settlement, Lazio, Italy [M. den Haan, 2013]The Grottarossa Pyroclastic Sequence (GPS) has been divided by Karner

(2001) into four subunits consisting of bedded gray to whitish gray Ignimbriteswith white scoria and pumice. Although identified by Karner (2001) the size of the geological unit was too small to be integrated into the 1:50.000 geologicalmap (Funiciello et al., 2008)The GPS Ignimbrite is less permeable compared to the overlying SKF, causing water to exfiltrate at the contact zone between the SKF and GPS at edges of theplateau.

PPT – Tufo Giallo di Prima Porta (circa 514 kya)Poorly lithified yellowish brown pyroclastic flow deposits form the

basis. The overlying second facies is a lithified yellow pyroclastic flow deposit with yellow pumice, rare gray scoria and occasionally large black scoria. The ignimbrite is mainly composed of the minerals leucite, sanidine, pyroxene and biotite. In the lithified facies sedimentary lithic clasts like unmetamophosed limestone and travertine can be found. Clast-rich flow in the initial stage (non-lithified facies) is followed by a fine-grained portion thatfilled topographically low areas as a blanket, indicating decreasing energy during the eruption.

TP- Tufo del Palatino, Alban Hills (528 kya) not visible on 1:50.000 geological map The Tufo del Palatino consists of two facies. The first is a lithified

dark gray lapilli-rich Ignimbrite. This main unit cannot be found within the Crustumerium site. The overlying unit is a light brown lapilli-rich ash fall deposit. The Tufo del Palatino has its origin in the Alban Hills volcanic complex.

TIB – Tufo Giallo della Via Tiberina (548 – 561 kya)Massive yellow ash fall deposit with scoria, lava, non-metamorphosed

sedimentary lithic fragments and white pumice. The TIB is fining upwards into apale yellow ash matrix with whitish-yellow leucite, sanidine and pyroxene crystals. The emplacement of this Igimbrite was responsible for damming and diverting the paleo-Tiber river north of Rome (Karner et al., 2001).

FCZ – Fosso Della Crescenza formation (middle Pleistocene 0.5 - 2.5 Mya)Conglomerates consisting of cemented fluvial deposits originating from

the paleo-Tiber river, with outcrops that can be found in the NNE section of the Crustumerium site.The Fosso Della Crescenza formation (FCZ) consists of gravels that are attributable to ariver environment (sub-rounded and sub-spherical). Above are slightly silty quartzitic sands, interbedded with clay and greenish-gray silt.



Figure 2. Detail of the 1:50.000 geological map presenting the geological unitspresent at the Crustumerium site (After Funiciello et al., 2008)



General introduction into Mediterranean soil formation

The Mediterranean climatic is characterized by strong seasonal contrast, with acool, wet season in which a precipitation excess occurs and the soil is leached, and a dry season in which the soil dries out and solutes may precipitate. This climate additionally leads to limited accumulation of organicmatter because of relatively rapid litter decomposition under such conditions. Weathering and leaching of the compounds released, such as basic cations and silica, are not very strong and lead to the formation of primary and secondary clay minerals of the kaolinite (1:1 clay mineral) and smectite group (2:1 secondary clay mineral group), while micaceous minerals present in the parent material (muscovite and biotite) are largely preserved in the form of illite and vermiculite, because of their stability (inherited clay minerals). Leucite minerals in the volcanic rocks play an important role in the pedogenesis of tuff soils because of their high weatherability, which makes aluminum easily available stimulating the formation of short-range-order aluminum phyllosilicate minerals (Collombo et al., 2007). Under Mediterranean climate, the most common secondary minerals formed by weathering of tuff formations under well drained conditions are the clay minerals halloysite and kaolinite and ferrihydrite/goethite (ironhydroxides) (Ezzaim et al., 1999). The volcanic rocks from the Monti Sabatini and Alban Hill volcanic complexes are very basic i.e. they have a very high potassium content and high potassium-silica ratio (Karner et al., 2001). During chemical weathering, an ongoing process, the easily weatherable minerals and materials (e.g. volcanic glass andleucite) are dissolved, the released silica and aluminium being recombined intohalloysite/kaolinite. This clay is leached into the lower parts of the soil where it forms illuviation cutans when the water evaporates. Upon prolonged illuviation, the illuvial horizon may start to act as a less permeable layer resulting in a less drained soil. Weathering under such relatively poorly drained conditions leads to the formation of smectites, i.e. of strongly swelling and shrinking clay whose properties enhance further stagnation. However, since volcanic rocks are mostly highly porous and losses by weatheringare large, enhancing the porosity of the weathering rock, clays mostly form under well drained conditions and thus largely consist of halloysite/kaolinite with smaller or larger ferrihydrite/goethite. Kaolinite and goethite form underrelatively warm climatic conditions and upon ageing of soils and thus are mostly encountered in older, deep soils in volcanic rocks.

If the weathering of the tuff is sufficient, significant amounts of halloysite and eventually kaolinite are formed, and residual iron and manganese accumulate. In freely drained soils, iron and manganese remain immobile. Because of the absence of organic matter iron and manganese are strongly bound to the clay. In the presence of organic matter, brown iron-organic matter-clay complexes will form, but such process only occurs in the humic topsoil. Below that topsoil, the soils have typically reddish brown to red colours. The process is described in French literature as rubefaction (Fedoroff, 1997).


Crustumerium settlement, Lazio, Italy [M. den Haan, 2013]As organic matter is being consumed by micro-organisms, the amount of oxygen inthe soil diminishes leading to the competition for oxygen. During the winter, in response to the soil moisture surplus and a lower porosity (Bt horizon), theredox potential may become high. Under such conditions, iron (and manganese) may be reduced and leached down the soil profile. During the summer the soil dries out, the iron and manganese become immobile and may precipitate. Such less well drained soils often show pronounced swell and shrink features (described as slickensides) .They may contain small black, rounded iron-manganese nodules (figure 3).

The sanidine, augite and feldspar minerals that are found within the tuff are calcium bearing, which is released when these minerals are weathered. The released calcium is dissolved and, like the iron-manganese concretions, may precipitate as calcium carbonate (CaCO3) concretions (figure 4) under poorly drained soil conditions. In soils with truly high groundwater, weathering products cannot move down the profile and smectite is formed. Such soils, if well developed, are classified as Luvisols and commonly have a calcic horizon. Where less prominent, soils are described as having ‘vertic properties’.

Halloysite and allophane clays are leached easily and illuviate, leading to therapid and strong formation of Bt horizons (t from the German ‘Ton’ or clay). Mediterranean soils with higher clay content in the subsoil (argic-B horizon) than in the topsoil due to clay translocation (i.e. illuvial accumulation of clay in the form of cutans) are generally classified as Luvisols. However, the clay-depleted topsoil may have been more or less eroded and therefore, the presence of an argillic B horizon (with illuviated clay) is sufficient for the identification of a soil as Luvisol. Moreover, the argic-B horizon may be very deep in which case the soils are classified as Nitosols, which however are relatively rare. Luvisols/Nitosols do not exhibit the swell and shrink featuresthat can be found in Vertisols. Soils that are less developed are classified asRegosols often complemented with vertic or vitric properties. If primary minerals and/or volcanic glass are present in the soil the suffix vitric is used. Vitric properties’ describe the presence of volcanic glass and other primary minerals derived from volcanic ejecta.

Lastly, the high potassium content of the volcanic and pyroclastic deposits in the fieldwork area is responsible for the high rates of mineral weathering. Thepotassium, when released as a solute, is directly available for nutrient uptake, giving these tuffs a high agricultural potential.

Fig 3: iron-manganese nodules (largest dimension ca. 6-7 cm)



Fig 4: calciumcarbonate concretions(largest dimension ca. 4-5 cm)



Soil formation within Crustumerium

In a characteristic and intact Mediterranean soil, the upper horizon is the dark A horizon, where the mineral and organic matter are intimately mixed. The depleted material accumulates in the Bt horizon, which can be recognized by itshigh clay content. The clay might even be visible as a film surrounding other minerals.. The B horizon ends where the unconsolidated parent material starts. This zone is called the C horizon and has a significantly lighter colour compared to the B-horizon. The depth of the contact zone between the B- and C-horizon will vary and depends on age of the soil and nature of parent material.

Since the introduction of mechanical ploughing the area is subjected to severe erosion. Ploughing eventually exposes the soil up to the bedrock resulting in erosion, primarily in the form of rill erosion, up to this level. Due to the rill erosion the soil is washed away several tenths of meters to be, temporarily, deposited. The lack of old, deep soils show that colluviation within the fieldwork area is limited, and most of the material is washed away from the fieldwork area.

Figure 5. Eroded topsoil directly above the lithified tuff of unit LTT with plough marks clearly visible on the tuff.


Crustumerium settlement, Lazio, Italy [M. den Haan, 2013]A thin layer of caprock of the ‘Fisheye’ tuff has presumably been identified within site U of Crustumerium. The unconsolidated ignimbrite consists of >30% leucite crystals with a diameter of 2-5 cm, which are easily recognizable. These highly eroded shallow soils are classified as Regosols with vitric and vertic properties.Soils within the geological unit NCF can be found in the south-eastern part of Crustumerium. The NCF consists of low energy fluviatile and lacustrine gravellydeposits, with low quality flint and limestone. The soils in this geological unit were formed when groundwater levels were higher in response to the high level of the paleo-Tiber. Weathering products could not be leached deeply into the soil and precipitated as iron-manganese and carbonate concretions. Nowadaysthese concretions are found in bands at the surface due to erosion. These (slightly) weathered soils are among the oldest and best preserved soils withinthe Crustumerium site. The soils are characterized by extensive clay illuviation, iron-manganese nodules and carbonate concretions. The intensive clay illuviation makes that these soils are classified as Luvisols.

The majority of the soils found in Crustumerium are on the geological unit LTT.These soils are heavily eroded and consist of a 40 to 60 cm thick organic rich Ap (p=plough) horizon directly above weathered tuff (figure 5). The LTT consists of more or less permeable layered bands with varying thickness. The impermeable tuff layers are responsible for the exuding of water and resulting erosion. More erosion resistant layers are clearly visible within the topography.The several layers within this unit clearly differ, some contain more scoria and pumice while others are more homogeneous. This makes it very hard for the untrained observer to make a distinction between colluvium and (weathered) tuff. Figure 6 shows the transition of the soil and the weathered tuff within one corehead. These soils are, due to their limited soil formation and thickness, classified as Regosols. When volcanic glass is present the prefix vitric is added. If soils are stable for some period of time minor clay illuviation might be found giving the soils the prefix vertic.

Figure 6. Transition between dark brown clayey soil and yellowish weathered tuff of the LTT unit containing many scoria and pumice.



Soil formation within the fine grained non-lithified yellow ash of the SKF unitis very limited. The majority of the soils within this unit are severely disturbed. The undisturbed soils show minor clay illuviation, classifying thesesoils as Regosols with vertic properties. The soils within the colluvial basin (coring X29) are classified as Luvisols, as they contained several layers of minor clay illuviation. The presence of multiple layers of clay illuviation indicate that colluviation did not occur as a single event, but during several periods after which soil formation took place.

The geological unit GPS has only been identified as bedrock. Therefore no corings were made in this unit and remarks on existing soils can therefore not be given. At the spring near the Fossa Formicola, south of the tombs, travertine like material has tentatively been identified at the excavated cave. Here the poorlylithified yellowish brown pyroclastic flow deposit of the PPT unit act as an aquifer, while the water stagnates on the ignimbrite resulting in the exuding of water. The hydrological properties of the PPT unit result in deeply weathered soils with relatively thick argic-B horizons. This makes that these soils are classified as Luvisols. Soils within the FCZ unit are very shallow and when on conglomerates are classified as Regosols or even Leptosols. Within the gray/green clay rich layers prolonged clay illuviation has resulted in the formation of Luvisols.

Throughout the fieldwork area and crossing several tuff layers a defensive ‘fossata’ characterizes the former occupational border of Crustumerium. This fossata is filled with local material strongly resembling weathered tuff, but the composition of the soil material is less dense en more chaotic when compared to the naturally weathered tuff. This makes that an observer with an untrained eye for identifying natural weathered tuff could easily make the wrong interpretation of the origin of the soil material. Pottery of several different periods and charcoal have been identified in the majority of the fossato fillings. The loose and chaotic soil structure in combination with a variety of pottery chards and charcoal could be used for the identification of the fossato filling.



Figure 7. Transition between a homogeneous tuff bedrock layer of the LTT unit (right) and chaotic ‘fossato’ filling (left).



Soil depth and sediment transport along transects

In addition to the geological background and corresponding soil forming processes of the area the correlation between surface find densities and sediment depths has been studied. The hypothesis is that surface finds concentrate where sediments accumulate; both would move down slope due to erosion and ploughing, and therefore lower terrain is expected to contain more dislocated archaeological material. In order to study the specific relationshipbetween the results of surveys and the subsurface a regularly spaced grid or transect of corings was designed. This would allow the creation and visualisation of a cross section of sediment depths and soil types on the settlement and to relate them to surface finds spatially. Besides transects, control corings were made at predefined locations that could potentially be of interest. Figure 8 shows the locations of the all corings made. A more detaileddescription of the findings is presented further on.

Figure 8. Locations of corings and corresponding transects within the Crustumerium site.

Transect X Transect X runs from south to north-east and captures the western side of the fieldwork area. The transect starts in a valley situated on the geological unitLTT. This valley is subjected to some minor colluviation. Due to erosion soil

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Crustumerium settlement, Lazio, Italy [M. den Haan, 2013]depths are limited and weathered bedrock is generally found within 50 cm. The former defensive trench, or fossato, has been identified at coring X3. Here archaeological materials such as pottery and charcoal were found at a depth of 130 cm. The fossato has been excavated in the bedrock and has later been filledresulting in the preservation of these materials. The transect continues in NNE direction. The morphology is characterized by twosmall hills, which have been cut by a colluvial valley. The top of the hills isunderlain by the geological unit NCF, as was observed by the changing bedrock color and presence of FeMn and CaCO3 concretions at the surface at coring X7 toX10 and X17 to X19 respectively. Soil formation in the fluviatile deposits of the NCF unit includes clay illuviation and formation of iron-manganese and calciumcarbonate concretions. During the summer cracks with a depth of 60 cm have been observed. Strong swell and shrink in combination with high clay content makes that these soils are classified as Luvisols. Combined with the presence of the iron-manganese (FeMn) and calcium-carbonate (CaCO3) concretionsat the surface indicate that these soils are old and (slightly) weathered. Soils in the valley consist of intermitting bands of more or less clayey material and are full of archeaological materials. The sequence and positioningof the claybands in the soil indicates lateral deposition (periods of colluviation) rather than vertical movement of clay within the soil. Due to thelimited inclination of the slope deposition rates exceed erosion resulting in adeep colluvial layer.The steep inclination of the slopes between X19 to X24 have resulted in erosionexceeding deposition, hence the absence of colluvium in the valley. The main road cutting through the Crustumerium area is crossed near the end of the transect. The morphology of the area has been altered for the construction of this road. Removed soil has presumably been deposited on the top of the adjacent hill, as a disturbed soil profile has been identified in coring X25 and X26.



Figure 9. Cross-section of Transect X showing depths of colluvium and bedrock

Transect YTransect Y starts in the south-east corner of the fieldwork area and runs in NNE direction to cover the eastern side. The chaotic and unorganized positioning of soil material in the first corings (Y1 to Y5) indicate that these soil depths are not related to natural colluviation, but rather to anthropogenic disturbance.Gray tuff bedrock of the GPS unit has been found at coring Y6 to Y8. Due to theimpermeability of this geological unit erosion is strong and has resulted in steep slopes with corresponding shallow soils. Corings Y10 to Y13, underlain bythe more permeable LTT unit, show rather stable soils. The protected position and permeability of the geological unit makes that erosion is limited, resulting in deeply weathered soils with bedrock depths exceeding 1 meter.Continuing along the transect soil depths of coring Y 14 to Y18 are limited to the plough depth (+- 40 cm). Corings Y19 to Y22 are underlain by unit NCF. This was confirmed by the presence of FeMn concretions and fluviatile gravels at the surface and in the soil profile. At coring Y21 the fossato, as previously described, was crossed again. An anthropogenic filling with pottery and charcoal defined the soil profile.The transect ends with Y24, which is underlain by the Fish-eye tuff of the VSN2a

geological unit as identified by the large (<1cm) leucite crystals. Again soil depth is limited to the plough depth.

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Figure 10. Cross-section of Transect Y showing colluvium depths and bedrock

Transect ZThe fieldwork area is crossed from west to east by transect Z. Apart from Z19, all soils are limited to the plough depth and bedrock is found within 50 cm.The soil depth at coring Z19 is related to the local preservation of colluvium.Down slope, at coring Z18, a band of the impermeable and more erosion resistantGPS unit has been identified. This band of bedrock reduces the inclination of the slope and therefore acts as a local catchment area for colluvium. The presence of two Bt-horizons indicate that the soil is relatively stable and soil forming processes have had some time to ‘mature’.

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Figure 11. Cross-section of Transect Z showing colluvium depths and bedrock


Crustumerium settlement, Lazio, Italy [M. den Haan, 2013]Corings summer 2012

During the summer campaign coring was limited as a result of the hard, dessicated clayey soils. Despite these difficulties results of the summer campaign are identical to those of 2013 . Therefore patterns in soil formation will be only briefly described.

Transect 1 coring 1 -12The first transect starts at the top of the hill at the end of the hollow or excavated road. Here soils are somewhat stable which can be seen by the presence of a thin Bt-horizon. Soils on the slope follow the normal trend of shallow soils subjected to serious erosion. The eroded soil material is partially and temporarily deposited in the valley bottom. The valley incision

at

coring 11 and 12 dissects a colluvial basin of which the boundary is situated near coring 9, here charcoal and pottery shards were identified in the soil profile.Figure 12. Simplified cross-section of coring 1 to 12 with absolute heights (m)of soil horizons as observed in the field.

Transect coring 13 – 28The second transect is in correlation with the previously describes transect X.At the start of the transect soil profiles are underlain by the NCF unit. Here clay illuviation characterizes the soil profiles directly underneath the ploughzone (+-40 cm). During the summer coring was limited to the illuviation horizondue to the hardness of the dry soil.

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Crustumerium settlement, Lazio, Italy [M. den Haan, 2013]On the slopes soil profiles are shallow and are merely limited to the plough depth before reaching bedrock. Coring 16 and 17 correspond with coring X13 and

X14 for the presence of intermitting bands of colluviated clay. The absence of colluvium in coring X21 and X22 is observed again in coring 22. Figure 13. Simplified cross-section of coring 13 to 28 with absolute heights (m) of soil horizons as observed in the field. N.B. coring 20 and 24 are excluded from the transect as they were only used as a reference.Transect coring 49 – 63The final transect covers the northern lobe of Crustumerium. Here soil profilesall follow the normal pattern as describes many times before.Coring 52, 53, 54 and 56 represent anthropogenic disturbances in the form of the fossato and/or other defensive works. This conclusion has been drawn by thehigh amount of charcoal and pottery chards in these soil profiles.

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Figure 14. Simplified cross-section of coring 49 to 63with absolute heights (m)of soil horizons as observed in the field.

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Crustumerium settlement, Lazio, Italy [M. den Haan, 2013]Description of several anomalies

During the fieldwork corings were made in several anomalies previously identified by the geophysical survey performed by Eastern Atlas (2010).

Anomaly C2, east of the deepened road near the incremented hill is a filled well. The anomaly has a circular form that cannot be explained by natural processes. Soil material consists of unsorted chaotic filling with an anthropological background, as can be seen by the presence of charcoal and pottery. Starting at 50 cm the soil becomes moist and mottling as a result of water stagnation becomes clearly visible in the soil profile. The combination of these findings led to the conclusion that the anomaly is a formerly used, filled well. This well most presumably has been dug to get access to the groundwater which stagnates on the underlying impermeable layer.

The anomaly near the electricity pylon, at site U of the Crustumerium fieldworkarea, consists of a tamped gravelly layer. Within this layer high amounts of very small (<1cm) pottery chards were found and the gravelly layer is filled with clay indicating that it is an old occupational layer. The pottery chards have been tentatively dated to be Roman.

The geophysical anomaly C3 is the result of a large gray tuff block in the soil. The exact size and positioning of this block have not been determined, but the soil material clearly deviates from the surrounding, natural situation.



Morphology of the Tiber valley

In addition to the soil survey of Crustumerium, a preliminary and investigativesoil survey was performed on the soil history of the southern side of the Tibervalley. The preliminary results are presented in figure 15 as a cross-section.

The floodplain of the Tiber river is enclosed by steep tuff plateaus. The transition zone between the floodplain and tuff plateau is characterized by a very steep layer of colluvium, consisting of eroded gravels originating from the FCZ unit within the tuff plateau. During pre-roman periods the Tiber river regularly flooded the valley resulting in a swampy environment. This can still be observed by the presence of lime gyttja. Closer to the Tiber river natural levees most likely have been formed. These levees prevented the water to runoff laterally, therefore presumably resulting in peat formation in the back-swamps of the floodplain.Large adaptations of the Tiber river during the Roman period changed the depositional environment, resulting in the deposition of post-roman gray clays.The floodplain is characterized by wetter and drier periods, which can be seen by the presence of land slugs throughout the profile (drier periods) and (pseudo)gley1 in the gray clay deposition (wetter periods). The position of the gyttja below the groundwater table increases the chance of finding well preserved pollen.

1 Pseudogley is used in international soil classification for identifying gray to blue-gray mottles of reduced iron (and manganese) as a result of stagnating water due to an impermeable soil horizon.


Crustumerium settlement, Lazio, Italy [M. den Haan, 2013]Figure 15. Schematic cross-section of the Tiber floodplain’s morphology. For a detailed description of the units see text above.



Conclusions

Soils in the Crustumerium area are shallow, mostly limited to the plough depth,and highly eroded. Soils on the top of the plateau are the most stable; this can be seen in the formation of a Bt-(argic) horizon. Weathering of the primaryminerals within the tuff result in the formation of clay. This clay is transported down into the soil profile to form a Bt-horizon. Soils that are more clayey and have a thicker argic horizon compared to others are therefore older. Clay illuviation also occurs in the graves at the Crustumerium site. Bands of clay illuviate on the closing stones and the bottom of the grave. The newly formed clay, consists mostly of halloysite, which is easily leached, leading to the filling of the tombs. Therefore clay illuviation is (partially) responsible for the preservation of archeological remains. The clay acts as a protective layer (seal) for the archaeological materials against the influence of water and oxygen and associated physical and chemical weathering of these materials.Depths of soil profiles on the slopes is merely related to the plough depth, but variation occurs within undulations at the scale of the slope itself. The eroded materials are deposited in colluvial valleys. The colluvium is rich in clay and therefore of importance as water regulator. The clay layer acts as an impermeable barrier and therefore regulates local groundwater levels and soil moisture. The distinctive morphology of the Crustumerium area is characterized by valley incisions, remnant plateau hilltops and colluvial valleys, which are mainly theresult of erosion generated by exuding water. Porous and permeable tuff layers act as an aquifer, and when groundwater comes in contact with less or impermeable tuff layers the flow direction becomes lateral. This results in seepage when the less porous tuff layer is close to the surface. Within Crustumerium seepage occurs at the contact zone between the yellow tuff (LPP/SKF). This process also occurs in between the different layer within the LTT, and whitish gray tuff (GPS). Due to the high local variability in water carrying capacities within tuff layers a clear and one-sided relationship between seeping water and transition to distinct tuff layers cannot be observed.

The volcanic rocks contain high amounts of primary minerals, which are transformed into clay through physical and chemical weathering. Clay minerals that are transported easily are lost making that subsoils are dominated by halloysitic clays. These clayey, halloysitic subsoils exhibit distinct seasonalswelling and shrinking . When dried out the soil cracks and these cracks are responsible for the deep penetration of water into the soil. When the amount ofrainwater exceeds the infiltration capacity of the soil, water will run over the surface and laterally through cracks resulting in erosion of the topsoil. The erosion is in the form of sheet and rill erosion (figure 16).Cracks in the clay can, besides erosion, cause the undermining of the superposition of archaeological artefacts. The artifacts can fall from the


Crustumerium settlement, Lazio, Italy [M. den Haan, 2013]surface into the 20-70 cm deep cracks placing them much deeper. Repositioning of archaeological artifacts can also be ascribed to erosion processes as rill erosion, depositing the artifacts several tens of meters further down the slope.

Figure 16. Sheet and rill erosion on the Crustumerium site (After Seubers, personal correspondence)

Coring allowed for anomalies to be described and explained with high certainty.Definitive answers on the meaning of some anomalies might be obtained with excavations and/or trenches.

Amounts of colluvium are rather limited, and the colluvium is mostly washed away. The relative young age of the soils would indicate that the majority of the erosion occurred since the start of mechanical ploughing. The relief of theoriginal landscape was much more dramatic, but due to the absence of the materials a reconstruction of this landscape cannot be performed. There is no generally applicable measure for the reconstruction of the original landscape. Such reconstruction can only be established on a local scale and for a certain area, and can only be semi-quantitative.Although erosion and ploughing smoothed the original dramatic relief and removed all evidence from historical soils. It is still to be expected that thearea of Crustumerium consisted of a terraced landscape, as still can be seen incultivated mountainous areas.

The preliminary survey performed at the Tiber valley showed scientific potential of this area. The fluviatile and lacustrine environment of the Paleo-Tiber valley could have resulted in the preservation of pollen. These pollen can, with the help of a paleo-ecological expert, give more insight into the (ecological) history of this area.





References

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Corazza A., Giordano G., de Rita D., 2006. Hydrogeology of the city of Rome. Geological Society of America special paper 408.

Ezzaim A, Turpault MP, Ranger J (1999) Quantification of weathering processes in acid brown soilsdeveloped from tuff Beaujolais, France/Part I. Formation of weathered rind. Geoderma 87: 137–154.

Federoff N., 1997. Clay illuviation in Mediterranean soils. Catena Vol. 28. Pp 171 - 189

Funiciello R., Giordano G., Mattei M., 2008. Geological Map of the Municipalityof Roma (scala /scale 1:50000). Pdf pp. 1-2

Karner D.B., Marra F., Renne P.R., 2001. The history of the Monti Sabatini and Alban Hills volcanoes: groundwork for assessing volcanic-tectonic hazards for Rome. Journal of volcanology and geothermal research 107. Pp. 185 - 219

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