Ice-pushed Ridges Research Articles

Historical parcellation and ridge-and-furrow relics of open strip-fields in the north-west European lowlands

ABSTRACT We tested to what extent ridge-and-furrow relics can be identified with airborne LiDAR imagery and analysed whether the ridge-and-furrow can provide an archive of historical parcellation dynamics in open strip-fields. Our case study area is the central-eastern Netherlands (Twente; Veluwe) and adjacent lowland Germany (Westphalia). We sampled eight mark territories containing twenty-one neighbourhoods with unurbanised open strip-fields. The sample contained coversand, ground moraine and ice-pushed ridge landscapes. The study was based on LiDAR-derived elevation models (DEM), historical cadastres and topographic maps, soil and geomorphological maps as well as an archaeological excavation. Ridge-and-furrow relics of 1–2 decimetres height, invisible to the naked eye, were detected in every strip-field. In large strip-fields, raised headland relics divided ridged beds into two shorter strip parcels. In afforested parts of strip-fields, ridge-and-furrow was generally better preserved. Ridged beds were broadly congruent with cadastral strip parcels from the early nineteenth century. However, cadastral strip parcels were often shorter than ridge-and-furrow beds but frequently several beds wide. The identified micro-topographic patterns turned out to be an archive of historical reparcellation dynamics.

The origin of alkaline fen in the Mosbeek Valley in the Netherlands is due to human impact rather than a natural development

Alkaline fens are important Natura 2000 habitats, which harbor many endangered plant species. Alkaline fens are formed in areas with groundwater discharge and usually developed in a natural way in the early Holocene. We radiocarbon dated the base of three peat deposits from spring fens along the flanks of the ice-pushed ridge near the village of Ootmarsum to find out when and why peat-forming vegetation started to grow. We cored a sequence in the Mosbeek Valley for detailed paleoecological analyses of micro- and macrofossils. To our surprise, we found strong evidence for human impact during the 13th and 14th centuries AD as the triggering factor for starting organic colluvial accumulation and peat growth at sites where natural springs are present. This shows that this fen is not a relic, but results from changes in land use. Human actions were: (1) deforestation causing increased run-off and reduced evaporation on the plateaus by the vegetation, resulting in increased seepage in the valleys, (2) intensification of agriculture, trade routes, and paired erosion, which formed colluvial deposits and sediment fans that hampered fast run-off water, (3) increased back and groundwater levels after the foundation of watermills; four lowering water levels due to intensification and reorganization of water use by new watermills, and (4) head cut erosion and spring erosion after privatization and cultivation of common pastures after the mid 19th century. This means that cultural-historical changes in the landscape were much more important for alkaline fens than expected.

Middeleeuwse ijzer- en houtskoolproductie op de Veluwe

Medieval iron and charcoal production in the Veluwe region Recent research on the iron production landscape in the coppice wood of ‘t Asselt near Rheden (Southeastern Veluwe) Between the seventh and twelfth century ad large-scale iron production took place in the Veluwe region (Central Netherlands) based on the extraction of iron-rich loam concretions in the regional ice-pushed ridges. This iron production required large amounts of charcoal which were produced by the large-scale burning of semi-natural and/or coppice woods. There is palaeobotanical evidence that the woodland vegetation of the Veluwe diminished by two-thirds in this medieval period, partly because of medieval woodland reclamations, partly because of charcoal-burning. A master thesis project revealed the existence of an extensive and largely untouched iron production landscape in the coppice wood of ‘t Asselt (Southeastern Veluwe). Based on ceramics from the eighth-twelfth century, historical-geographical relics as well as written evidence this early and high medieval landscape layer seems rather complete and untouched, which makes it a very promising site for further archaeological research on medieval iron production landscapes.

Eemian to Early Weichselian regional and local vegetation development and sedimentary and geomorphological controls, Amersfoort Basin, The Netherlands

Abstract Two new records from the Amersfoort glacial basin are investigated by means of pollen analysis. The cores are situated in the deeper part, close to the original Eemian stratotype Amersfoort 1 (Zagwijn, 1961) and at the margin of the basin. The aim is to reconstruct the Eemian and Early Weichselian vegetation development and to explore the impact of accommodation space, influx of allochthonous pollen and geomorphology on the vegetation composition. The results of the Amersfoort Basin are compared to the current Eemian stratotype in the Amsterdam Basin and other Eemian sites in the Netherlands. An almost complete Eemian to Early Weichselian sequence (E2-EWII) was retrieved from the deeper part of the Amersfoort basin. The late Saalian (LS) to early Eemian transition is not recorded in the Amersfoort basin, in contrast to the deeper Amsterdam Basin. The basin marginal core Den Treek reveals a condensed late Eemian (E5-6) and Early Weichselian (EW I-II) succession showing the importance of accommodation space. The first impact of the Eemian transgression is registered at the E3 to E4a boundary in the Amersfoort and Amsterdam basins, and highest sea level is proposed at the end of pollen zone E5. Upstream in the Eemian delta, in the palaeo-Vecht valley and IJssel Basin, the transgression is recorded later. The influx of reworked (allochthonous) pollen in clastic sediment units hampers vegetation and climatic reconstructions during the LS and Eemian. The early appearance of Picea in zone E4 and Abies in zone E5 in clastic sediment intervals can be related to long-distance transport by the river Rhine and redistribution in the Eemian delta. Local vegetation development can complicate regional biostratigraphic correlations. Alnus, considered characteristic for the late Eemian (E5-6), shows large differences over short distances in the Amersfoort Basin, related to local alder growth since Eemian E3. Carpinus, diagnostic for pollen zone E5, shows high values in the basins adjacent to higher, well-drained ice-pushed ridges, but low values in low-relief environments. Salt- to brackish-water marshes were present during high sea level in zone E5 in the Amsterdam and Amersfoort basins, while further upstream in the Rhine delta brackish to fresh-water tidal conditions dominated. In line with Zagwijn (1961), the E6 to EWI boundary is defined at the first opening of the vegetation cover with Calluna, Poaceae and Artemisia increase, often coinciding with a lithological change from organic to clastic deposition, reflecting increased landscape instability. The cores from the Amersfoort basin reveal a complete Eemian to Early Weichselian record. It is suggested to define the boundary stratotype for the base of the Weichselian Stage in the Amersfoort Basin. The current stratotype Amsterdam-Terminal reveals a fully developed LS to Eemian transition and contains the boundary stratotype for the base of the Eemian Stage.

Maximum Holocene groundwater levels and associated extension of peat in the border zone of ‘Het Gooi’ (the Netherlands): a reconstruction based on the study of soil transects

Abstract The area ‘Het Gooi’ in the Netherlands is part of a Pleistocene ice-pushed ridge system that partially drowned during the Holocene upon sea level and associated groundwater rise. As a result, the ridge system was gradually encroached by peat. From the late Middle Ages onward, man reclaimed the peatlands surrounding Het Gooi, heavily reducing their extension and lowering the regional groundwater level by increasingly intensive drainage. Based on historical and archaeological arguments, several authors assume that the Holocene peat cover in the border zones of ‘Het Gooi’ formed the extension of large raised peat bogs that formed further to the west and east, respectively. They presume that in the late Middle Ages these extensions reached ‘upslope’ to a maximum altitude of 3 m + NAP (Dutch Ordnance Datum – approximating mean sea level). However, the original extension is difficult to reconstruct, as this peat has disappeared as a result of its exploitation and oxidation, if having been present at all. In this study, the maximum extension of the Holocene peat cover on the ice-pushed ridge system was reconstructed based on soil characteristics. Used soil characteristics concerned the presence of iron coatings around sand grains and the upper boundary of gleyic features, because these are indicators for the mean highest groundwater level (MHG). For peat to form, this MHG needs to be at or just above the ground surface for most of the year. Based on study of a number of soil transects, we reconstructed to what maximum altitude peat encroachment may have occurred. This ‘maximum extension’ can alternatively be described as the maximum altitude of the bottom of the peat onlapping the ridge system. In the western border zone, this peat cover was found to have reached to c. NAP or just above, near Hilversum. No indications were found for the occurrence of raised bogs. We conclude that the phreatic groundwater level in this zone was controlled by the sea level and associated lake levels (Naardermeer and Horstermeer), a dominant role being played by the shallow presence of Pleistocene formations with a high hydraulic conductivity. In the eastern border zone, altitudes were more variable and in places reached 2 m + NAP. Peat at this higher elevation probably formed under the influence of a higher phreatic groundwater level, induced by the presence of a clayey Eemian fill with low hydraulic conductivity in the adjacent glacial basin (the Eem valley). This study demonstrates the value of detailed soil transect studies for palaeogeographical reconstructions of the former Holocene peat cover in Pleistocene landscapes of NW Europe. It also provides independent data for validation of geohydrological models for such landscapes.

Pleistocene glaciation of Fenland, England, and its implications for evolution of the region.

Detailed investigation of landforms and their underlying deposits on the eastern margin of Fenland, East Anglia, demonstrated that they represent a series of glaciofluvial delta-fan and related sediments. Associated with these deposits are glacially dislocated sediments including tills, meltwater and pre-existing fluvial sediments. These ‘Skertchly Line’ deposits occur in the context of a substantial ice lobe that entered Fenland from the N to NE, dammed the streams entering the basin and caused glacial lakes to form in the valleys on the margins. Bulldozing by the ice lobe caused a series of ice-pushed ridges to form at the dynamic margin, especially at the ice maximum and during its retreat phases. Meltwater formed a series of marginal fans that coalesced into marginal accumulations in the SE of the basin. The ice lobe is named the Tottenhill glaciation. Further investigations of the Fenland margin have revealed the extent of the Tottenhill glaciation in the Fenland Basin, to the south and west, in sufficient detail to demonstrate the nature of the Tottenhill ice lobe and the landscape left on deglaciation. The ice lobe is likely to have been prone to surging. This is indicated by the low gradient of the ice lobe, the presence of underlying ductile Mesozoic clays, the evidence of ice-marginal flooding and the presence of arcuate glaciotectonic push moraines. Regional correlation, supported by independent numerical geochronology, indicates that the glaciation occurred ca 160 ka, i.e. during the late Middle Pleistocene, Marine Isotope Stage (MIS) 6, the Wolstonian Stage. Comparison and correlation across the southern North Sea Basin confirms that the glaciation is the equivalent of that during the Late Saalian Drenthe Stadial in The Netherlands. The implications of this correlation are presented. Before the glaciation occurred, the Fenland Basin did not exist. It appears to have been initiated by a subglacial tunnel valley system beneath the Anglian (=Elsterian, MIS 12) ice sheet. During the subsequent Hoxnian (=Holsteinian; approx. MIS 11) interglacial, the sea invaded the drainage system inherited following the glacial retreat. The evolution through the subsequent ca 200 ka Early to Middle Wolstonian substages, the interval between the Hoxnian (Holsteinian) temperate Stage and the Wolstonian glaciation, represents a period during which fluvial and periglacial activity modified the landscape under cold climates, and organic sediments were laid down during a warmer event. Palaeolithic humans were also periodically present during this interval, their artefacts having been reworked by the subsequent glaciation. The deglaciation was followed by re-establishment of the rivers associated with the deposition of Late Wolstonian (Warthe Stadial) gravels and sands, and later, deposits of the Ipswichian interglacial (=Eemian, approx. MIS 5e) including freshwater, then estuarine sediments. Subsequent evolution of the basin occurred during the Devensian Stage (=Weichselian, MIS 5d-2) under predominantly cold, periglacial conditions.

Gravel size matters: Early Middle Palaeolithic artefacts made from local Rhine and Meuse deposits in the central Netherlands

AbstractThe artefact size of the Early Middle Palaeolithic (EMP) assemblages in ice-pushed Rhine–Meuse deposits in the central Netherlands decreases northwestward. This trend correlates to the downstream fining direction of the Rhine–Meuse fluvial system, the source of the rock material, showing that locally available material was used. Furthermore, also in line with the fluvial trend, the gravel and cobble trends indicate that EMP artefacts could be present in the buried part of the ice-pushed ridges in the northern part of the central Netherlands (southwest Flevoland). Based on the gravel and cobble data, combined with the literature study, we argue that the artefacts date to marine isotope stages (MIS) 7–6. The review of published data shows that during the time of deposition of the artefact-bearing layer (early MIS 6), the Rhine–Meuse study area was part of a braidplain located in a slightly incised valley. To the north a polar desert was present.

The Middle Palaeolithic of the Netherlands – Contexts and perspectives

We present a concise overview of Middle Palaeolithic research in the Netherlands. The area, which is situated along the northwestern edge of the known Neanderthal world, is very divers in terms of geological development. As a result of glacial cycles, hominin occupation can be characterised as intermittent and probably sparse. Well-preserved sites are primarily known from the loess region in the southern part of the province of Limburg and to a certain extent from the Roer Valley Graben. Further to the north artefacts do occur ‘geological in situ’, i.e. in the ice-pushed ridges in the central Netherlands and on the Drenthe–Frisian till plateau in the northernmost part of the country, but primary archaeological in situ situations have yet to be discovered. The oldest traces of occupation date to MIS 9 or MIS 7 (Belvédère quarry and the ‘Rhenen Industry’), the youngest (stray finds) can be attributed to the Blattspitzengruppen and the Lincombian-Ranisian-Jerzmanowician (MIS 3). The (Upper) Acheulian, several Mousterian variants and the Keilmessergruppen are also attested. This paper serves as a starting point for future research.

Fluvial evolution of the Rhine during the last interglacial-glacial cycle in the southern North Sea basin: A review and look forward

This paper presents the current state of knowledge on the evolution and depositional history of the River Rhine in the southern part of the North Sea basin during the upper Middle and Late Pleistocene, and its response to climate change, sea-level oscillation and glacio-isostasy. The study focuses on the development of the Eemian interglacial lower-delta in the central Netherlands and its relation to records of climate and sea-level rise, and uses the Saalian and Weichselian pre- and postdating periods to place its development in context.The Rhine fluvial system fills the gradually subsiding North Sea basin, but its development has strongly been affected by the Saalian glaciation and its remaining topography. Ice-pushed ridges originating off the limit of maximum glaciation basically divided the central Netherlands into two sedimentary depocentres: a central depocentre within the former ice-limit, and a southern depocentre south of it.The sedimentary record of the central depocentre, including an incised-valley fill, shows a 20–40 m thick stacked sequence consisting of three units. The incised-valley fill consists of a Late Saalian to early Eemian age lower fluvial unit and a Weichselian age upper fluvial unit, both composed of coarse-grained channel deposits. Sandwiched in-between is a 5–15 m thick record composed of fine-grained fluvial and estuarine (tidal) floodbasin and shallow-marine deposits. It is of Eemian interglacial and Early Weichselian age, and comprises transgressive and highstand deposits that show the drowning of a fluvial system. Inland parts transformed from fluvial to deltaic and estuarine environments, and the most downstream parts transformed to a shallow-marine embayment. Preservation of these units occurred, despite considerable sea-level fall and climate-controlled erosion taking place in the last-glacial. Preservation potential was increased by the fact that the Rhine system avulsed away to the southern depocentre, halfway the Weichselian Pleniglacial. Consequently, the infill of the southern depocentre is of an entire different nature, and last-interglacial transgressive or highstand units are hardly preserved.Because of glaciation and resulting depocentre configuration, the Netherlands in NW Europe thus offers a very good opportunity to study the transgressive interglacial lower-deltaic records and falling-stage preservation thereof – both key elements for understanding sedimentary development over full 100-ky glacial-interglacial cycles of climate and base-level change.

Influence of palaeokarst morphology on the formation of ice-pushed ridges: a case study from Rejowiec, eastern Poland

This article describes the well-exposed landform/sediment assemblage of a Pliocene palaeouvala and Pleistocene (Saalian) ice-pushed ridge in a large quarry in Upper Cretaceous marls near Rejowiec, eastern Poland. The site provides a rare opportunity to study the development of glacial depositional and deformational processes on morphologically and structurally variable bedrock. The internal structures of both chronostratigraphic units which comprise this assemblage provide evidence of two principal stages of deformation associated with an ice advance on a karstified foreland, each stage preceded by stabilization of the ice mass. The first deformational stage was characterized by the development of imbricate structures in outwash sediments (the initial ice-pushed ridge) and listric faults at the contact between the bedrock and palaeouvala fills (caused by an increasing ice load). Shallow subglacial folding of the proximal fan deposits, and a listric thrust which limited the extent of the ice-pushed ridge under development occurred in the second deformational stage. The data assembled indicate that the formation of push moraines (in a general sense) during ice-sheet advance is controlled by the substratum undulations. Closed depressions with loose fills in the foreland of advancing ice sheets can control the deformational style, geometry and extent (both lateral and vertical) of the push moraines.

Plant remains from the Kreftenheye Formation (Eemian) at Raalte, The Netherlands

The Kreftenheye Formation is a fluviatile deposit occurring in the central part of the Netherlands. The present study deals with the plant remains found in suction-dredged bulk samples from its largely Eemian Zutphen Member, a clayey sediment formerly known as the clay–peat bed or the Salvinia bed. It reports the discovery of a large number of galls induced by the asexual generation of the currently southeast European oak gall wasp Andricus hungaricus. The associated plant remains include seeds, fruits, conifer cones, wood, spines, leaf fragments, buds, bud scales, stem fragments, bryophytes, sporangia, oospores, palynomorphs, galls, jet and amber. The palynomorph samples indicate the Late Eemian pollen zone E5b of the Carpinus phase as the origin of the galls, but the other macrofossils may have a wider stratigraphic range, including possibly also material from (a part) of the zones E3–E5a. Several elements, such as Celtis, Chamaecyparis, Juglans, Magnolia and Pterocarya, date almost certainly from earlier interglacials. The list of plant remains is dominated by fresh water plants, riparian elements and ancient woodland taxa, which reflects the ecological range of the IJssel valley and adjacent ice-pushed ridges very well. Thermophilous elements include Brasenia schreberi, Buxus sempervirens, Cornus mas, Najas minor, Salvinia natans, Trapa natans and possibly also Andricus hungaricus. Quercus cerris, the probable host of the sexual generation of A. hungaricus was not found.

Influence of palaeokarst morphology on the formation of ice-pushed ridges: a case study from Rejowiec, eastern Poland

Influence of palaeokarst morphology on the formation of ice-pushed ridges: a case study from Rejowiec, eastern Poland

Late Saalian and Eemian deposits in the Amsterdam glacial basin

Abstract During its maximum extension, the Saalian ice cap reached into the central Netherlands, where glacier tongues excavated over 100 m deep basins in the unconsolidated Middle and Early Pleistocene sediments. The basins are filled by relatively thick successions of Late Saalian, Eemian and Weichselian sediments. The fill of the Amsterdam glacial basin is among the best known and studied in the Netherlands. The Late Saalian sediments consist mainly of warves and ill-bedded clays and silts with, along its southern margin, influxes of sands from the surrounding ice-pushed ridges. During deposition of these sediments, the Amsterdam basin formed part of a large lake extending into the present North Sea. Draining of this lake at the end of the Late Saalian left small, shallow pools at the site of the glacial basins. Late Saalian and Eemian sediments are probably separated by a short break, although sedimentation may have been continuous in the deepest part of the basin. The Eemian deposits consist in main lines of a thin, diatom-rich sapropel at the base, overlain by an up to 30 m thick clay-rich sequence covered by a wedge of sand that measures more than 20 m in the northern part of the basin and that peters out southwards. As appears from the fauna, most of the clays were deposited in a lagoonal setting shielded behind a threshold and/or barrier. The rate of sediment supply was low so that lagoonal conditions were maintained over a long timespan. Sands derived from the surrounding ice-pushed ridges and transported by longshore drift and tidal currents formed a spit at the northern margin of the basin, which moved southward after eustatic sea-level rise stabilized and the lagoon was filled by clay. Loading of this clay-rich sequence by the spit and its washover fans induced subsidence, however, because of compaction, so that marine conditions were maintained until after the Eemian highstand. Fluvial and eolian sediments of Weichselian age, locally reaching a thickness of almost 10 m, eventually levelled the Amsterdam glacial basin.

The Eemian stratotype locality at Amersfoort in the central Netherlands: a re-evaluation of old and new data

Abstract In order to obtain a better understanding of the infilling of the Saalian glacial basins during the Eemian, particularly following the recent research in the Amsterdam Basin (Terminal borehole), it was necessary to re-investigate the type locality of the Eemian at Amersfoort. Both published and unpublished data from various biota (diatoms, foraminifers, molluscs, ostracods, pollen) provide new information on the changing sedimentary environments during the Eemian. Although the organic and clastic sediments of the infilling represent nearly all the pollen zones, the sedimentary sequence at Amersfoort is discontinuous: four breaks at least are recognised at the type locality. The successive sedimentary environments and the breaks in the record are linked with the transgression of the Eemian sea, the topographic position at the margin of an ice-pushed ridge, and the changes in hydrodynamic conditions. Local conditions, such as a sandy sea bed, shallow water and a reduced water exchange near the North Sea margin, influenced the salinity of the basin. Rib counts of Cerastoderma edule shells indicate a higher salinity at the end of the Taxus (E4b) and the beginning of the Carpinus (E5) zones than that present in the modern North Sea. Local conditions were responsible for the higher salinity following the climate optimum. During the Abies phase (the later part of regional pollen zone E5), the sea level had already fallen. The change from eu-trophic peat growth (with Alnus and Salix) to an oligotrophic Ericaceae/Sphagnum community at the end of the Eemian resulted from the change from a marine to a fresh-water environment, probably coherent with a deterioration of the climate.

Imaging groundwater ‘steps’ in push moraines by georadar

Abstract Georadar has proved to be a valuable new geophysical technique for non-destructive groundwater exploration in the Netherlands, yielding detailed continuous images to depths of 40 m under favourable conditions. During the Saalian, glacial action in the Netherlands deformed the fluvial deposits and created ice-pushed ridges or push moraines. Most of these displaced fluvial sediments are coarse-grained sands and gravels, but thin clay layers which acted as sliding planes during the glacial deformation are also present. The ice-pushed ridges are infiltration areas and the thick coarse sediments are important aquifers of the Netherlands. To be able to adequately manage these groundwater resources and forecast their depletion and contamination, the groundwater flow must be modelled in detail. Steeply dipping clay layers impede groundwater flow and create ‘steps’ in groundwater levels. Georadar surveying enables water tables to be imaged precisely and detects the exact location of such groundwater steps and vertical barriers. During a georadar survey near the village of Epe on one of the major ice-pushed ridges of the Netherlands, a sudden and spectacular offset in the water-table reflector was observed. The jump in the reflector could be traced along a number of parallel lines. A series of nine boreholes drilled to verify the georadar results confirmed the presence of important steps in the water table and proved the georadar predictions on depths and offsets to be remarkably close to reality.

Hydrology of the wetland Naarderemeer: Influence of the surrounding area and impact on vegetation

The Naardermeer is a polder located in the central Netherlands at the foot of a sandy ice-pushed ridge. It has been put on the list of.internationally important wetlands. Groundwater flow is directed from the ice-pushed ridge in the east towards the river plain in the west. Seepage from the ice-pushed ridge occurs in the eastern part of the Naardermeer while in the western part infiltration occurs as a result of low water levels in the adjacent polders. During the last decades the water levels in the Naardermeer have become steadily lower due to increasing groundwater extractions in the hill ridge and lowering of the water levels in the adjacent polders. External surface water is suppleted threat to compensate water deficits in summer. These changes form a possible threat to the preservation of the botanical significance of the Naardermeer. In 1985 a survey was started to establish the relation between hydrology and the occurrence of plant species in the Naardermeer. The hydrological data shows that water quantity and quality in the Naardermeer are strongly influenced by the surrounding area. As concerns quantity, the water input by seepage is inadequate to compensate for water loss through infiltration. This results in a mean net infiltration of 0.4 mm/day. The influence of the surrounding area also emerges from the quality of the groundwater. In the seepage area groundwater is generally of the calcium-bicarbonate type. In the most eastern part of the seepage area groundwater occurs with high nutrient load originating from polluted surface water just outside the Naardermeer. Infiltration recorded from piezometric data was confirmed by the presence of water with rain water attributes in the upper groundwater layers. In the area a clear relationship exists between the pattern of hydrological attributes and the vegetation pattern in reed marshes. Vegetations related to the quality of rain water occur in a random pattern in the Naardermeer. Vegetations related to water rich in calcium and bicarbonate occur in the seepage area where they are supplied directly by groundwater of the calcium-bicarbonate type. Furthermore, they occur in the infiltration area where they are supplied by surface water. The surface water originates from groundwater in the seepage area which flows into the infiltration area. This seepage driven flow will prevent the suppletion water from spreading all over the Naardermeer. This is important as most of the species present respond negatively to the quality of the suppletion water. This study illustrates the importance of hydrological information for the conservation of nature in wetlands. The impact of both internal and external (upstream and even downstream) processes has to be considered.

Surface Water Quality in Relation to Soil Type, Land use and Discharge in a Rural Catchment Area

Surface water quality and the factors that influence it were investigated in a rural area that lies between an ice-pushed ridge and river backlands. The influences of soil type, land use and household wastewater on the concentrations of nutrients were studied during base-flow as well as peak-flow periods. Samples were taken in ditches and canals, more frequently as discharge increased. Average values of the base-flow quality in ditches were calculated per soil type and land use and for all measure points in the canals. The fluctuations of water quality were plotted and analyzed in concentration-time and concentration-discharge diagrams. The relative importance for the quality of total discharge in the main canal of the loads coming from several parts of the drainage basin, was investigated with a stepwise multiple variable regression analysis. During base-flow situations sources of nutrients are the intensively cultivated sandy soils. In addition the concentration of orthophosphate is increased by household wastewater. During peak-flows, potassium and nitrate run off from all soil types with all types of land use, resulting in high concentrations in the surface water, in spite of dilution by rainwater. It is concluded that for proper management, intensity of manuring and fertilizing should decrease, especially on sandy soils. Wastewater should be drained outside the basin, to lower the concentration of orthophosphate.

A model of glacial tectonics, applied to the ice­pushed ridges in the Central Netherlands

During a detailed structural geological and geomorphological survey of ice-pushed ridges around the Gel­derse Vallei (centre of the Netherlands) several questions arose about the origin of these landforms. The Gelderse Vallei is a Saalian glacial basin filled with younger sediments, 40 km long and up to 20 km wide, running NNW-SSE. Thrust sheets which build up the ice-pushed ridges on either side of the valley were transported away from the centre of the basin. They are up to 25 m thick and are pushed to a level about 100 m above the decollement. The thrust sheets, mainly consisting of coarse sand layers, moved as rigid masses, while only the finegrained basal layers deformed by heterogeneous simple shear. Pore water pressure plays an important role by greatly reducing the sliding friction in the decollement layer. Once a glacial thrust sheet is formed, the weight of the upward moving frontal part (the toe) offers a resistance to movement, which can not be overcome by the basal shear stress of the glacier alone. Basal shear stresses in most glaciers are in the order of 0.1 MPa (1 bar) and appear to be incapable of lifting a toe of the size found in most ice-pushed ridges in the Netherlands. The article presents a model of a system ice lobe - substratum, which is to a large degree controlled by gravitational forces. The dilemma may be solved by the concept of the gradient stress field under the marginal area of an ice lobe. This concept was first formulated by Rotnicki (1976). Because ice thickness decreases towards the margin, the substratum is subjected to a decreasing load in the same direction. If the increments of stress difference under a slab of ice in the marginal zone are summed, a gradient stress is arrived at which is di­rected towards the ice edge. An estimate of the forces involved in the static equilibrium around the glacier margin indicates that the contribution by the gradient stress field is sufficient to move the toe. In the model proposed in the article, the energy to move and imbicate glacial thrust sheets around the margins of an ice lobe, is supplied by the continuous flow of ice into the ice lobe. The dimensions of ice­pushed ridges are thus to a high degree functions of the thickness of the ice lobe or icecap. The Gelderse Vallei ice lobe will have been at least 250 m thick to be able to form the ridges. The basal shear stress contributes less to the tectonic transport, the larger the thrust sheets and the high­er the ice-pushed ridges.

Sandy-gravelly mass-flow deposits in an ice-marginal lake (Saalian, Leuvenumsche Beek Valley, Veluwe, the Netherlands), with emphasis on plug-flow deposits

In three pits in the Leuvenumsche Beek Valley (near Ermelo and Elspeet), massive and diffusely banded clean gravelly sands, found in association with sandy sediments showing some similarities to the Bouma sequence, were interpreted as originating from sediment gravity flows. The flows, which came to rest on a ca. 6° paleoslope, probably originated from slumping of parts of the ice-pushed ridges at either side of an ice-marginal lake. Major characteristic features in the three sand pits studied are U-shaped channel forms with a maximum depth and width of 10 × 25 rmm. The fills consist of sand and gravels, locally containing giant sand-clasts. The sediments just below the base of these channel-forms are commonly distorted and folded. Especially the massive more sandy fills are surrounded by a finer-grained diffusely bedded and inversely graded zone. These sedimentary features suggest that “rigid” sediment plugs sheared downslope, generating a finer-grained shear zone around the plug scouring a slide scar till its present semi-cylindrical form and preserving the steep sides (exceeding the angle of repose) of the channel-form by “freezing”. The U-shaped channel forms (plugged troughs) are locally overlain by shallower gently sided trough structures, with mainly diffusely banded infillings. Very shallow and often smaller troughs are found again on top of these massive and diffusely banded infillings. The sediments in these troughs consist of repetitions of two types of microsequences: (1) thickly bedded sequences of normally graded massive sands, near-horizontal stratifications, plane beds and cross-bedded sets; and (2) thinly bedded sequences of relatively thick units T A and thin units T C and T D of the Bouma sequence. The repetitions of the microsequences are interpreted as flow pulsations. Upwards thinning of the sequence, together with a finer grain size may point to waning flow conditions. Their association with plugged troughs is suggestive of retrogressive flow-slides.

The spatial distribution of polluted groundwater from rural centres in a recharge area in the Netherlands — The Veluwe

A topographically elevated area of ice-pushed ridges was selected for a study on groundwater chemical composition in relation to the potential pollution sources. A groundwater sampling technique using small ceramic cups was adapted for this study. This technique allows detailed sampling in depth for each borehole, even in areas with deep groundwater levels. Results of the chemical analyses show great variations in vertical distribution of the ion-concentrations in groundwater. These variations reflect the differences in land use and are also shown to be a function of the groundwater flow patterns.