GEOLOGY AND MORPHOLOGY OF A PART OF THE OOTMARSUM ICE-PUSHED RIDGE M.W. van den Berg* and C. den Otter** CONTENTS I. INTRODUCTION 2. GEOLOGICAL SETTING 3. INTERNAL STRUCTURE OF THE OOTMARSUM RIDGE 4. MORPHOLOGICAL EXP RESSION OF THE LITHOLOGY 5. ABSTRACT 6. ACKNOWLEDGEMENTS 7. REFERENCES *Soil Survey Institute and State Geological Survey, P.O. Box 98, 6700 AB Wageningen, the Netherlands **State Geological Survey, P.O. Box 58, 7240 AB Lochem, the Netherlands 35 VAN DEN BER.G DEN OTTER 36 M.W. & c. I. INTRODUCTION 2. GEOLOGICAL SETTING The finding of some artefacts on the western The find-spots of the artefacts are located on flank of the Ootmarsum push moraine ridge the western flank of a ridge that forms part of (Stapert, this issue) gave rise to a wish for a series of elevations that continues in a more insight into the geological setting of the northern direction, across the Dutch-German area concerned. border, into the county of Bentheim (fig. I). The Mapping Department of the State This ridge consists primarily of non-glacial Geological Survey carries out a systematic deposits, and was formed by ice-pushing in survey of the whole country. The results are the Saale period. The morpho-genetic pro­ published in a series of maps on a scale of cesses which have been of basic import­ I: 50,000. During the last days of the survey ance for the ultimate situation as we know for the sheets AlmeloJ Denekamp (28 E-29 it today can be divided into three groups: W), the above-mentioned finds were dis­ a) During the last stadial of the Saalian covered. These, as yet unpublished, survey Stage (fig. 2) (Zagwijn, 1973), the inland ice results form the basis of this short summary of sheets slided over a frozen subsoil, causing the geological outlines. More extensive data little disturbance of the subsoil. Periodically, will be published in the memoir to the sheet however, the glacier lobes of the same ice 28 E-29 W (Van den Berg, in prep.). Geomor­ sheet were able to cut deeply into the pre­ phological data are taken from an earlier glacial deposits and push them up into ridges. published Geomorphological Map of the This mechanism is not yet fully understood, Netherlands, sheet AlmeloJ Denekamp, scale although explanations have been offered by I: 50,000. Zagwijn (1975) and Jelgersma and Breeuwer tentative m.an lumm,r temperature doling O· S· 10· IS· 20· subdivision Iyears 8,PJ of Iht conil. tundra fOll.l polOfotilhic 50.000 • •• II��' ,"f -l:., ..... -; ...... r - --' �___I �:1-7' '-..<:L :---J.. / / / I ...- /'/ / I y' / / V 1/ w / I z \ / / I c o ',[ / I u 100.000 E o Ql .ALMELO W o o • BENTHEIM • HENGELO 150.000 c o 10 km o L..-_--'-_-.Jf (/) 1--.:)lllZjd<;�d3D4. .. "" 1 1151::;.)161-17 200.000 Holst.inion • • Fig. 1. Physiographic picture of sheet 28E-29W and the adjacent county. of Bentheim. 1. push direction of the �'I Qdi Qh Saalian land ice lobes; 2.push moraine; push moraine, DinlHllodials 3. orinltr9tacials eroded; 4. Dutch-German border; 5. mapsheet Almelo/ Denekamp; Saalian outwash fans, partly covered by Fig. Climatic curve for the Late Pleistocene in the 6. 2. younger deposits; 7. section shown in fig. Netherlands (based on Zagwijn, 1975). 3. Geology and morphology of a part of the Ootmarsum ridge 37 w E 80 E 70 • • • • • • • • • 60 \ 50 ===== PUSH DIRECTION ',\ \ \ \ 40 E \ \ \ \ \ '\ 30 '\. \" \ c ,\ \ 20 \ � \ \ \ ...... \ \ _ ____ _________ ___ _ __ ,,",--.'�'.'�':".''-U. \ \ \ \ \ 30 -- - ." . 40 : 50 60 i Site Middle Paleolithic 70 Artefacts o 2km ' L' __---'- __.....J 1 80 Fig. Schematic cross section through the Ootmarsum push moraine ridge. A. fine-grained Tertiary deposits; B. coarse­ 3. grained Middle Pleistocene deposits; C. ice-pushed strata; D. Saalian outwash fan; basal till of the Saalian ice sheet; F. E. Weichselian soliOuction fans. (1975). The direction of push was, for the N-S map a very regular pattern is found on the section of the ridge, from the east; the E- W Ootmarsum ridge. This regular pattern is the section in the county of Bentheim has been response to the drumlinoid form of this part pushed up from the north (fig. I). of the ice-pushed ridge. This form is caused During the formation of the ridge, I by the overriding ice-sheet. glacial meltwater flowed over it, taking up A detailed description of the periglacial local material from the ridges, and thus processes and relat�d forms is given by many building up from this a major outwash fan on authors (e.g. Van der Hammen et al., 1967; the western flank (figs. I and 3). After the Maarieveld, 1976). Aeolian accumulation formation of the ridge and the outwash fans, forms are well known from this timespan, they the ice sheet flowed further south, meanwhile occur mainly in the lower areas between the overriding and strongly eroding the ridges; a ice-pushed ridges, and are very scarce on the basal till or the remains of it are found all over ridges themselves. Coversand accumulation is the ridges, and on the outwash fan. found on top of the solifluction fans that have b) Major erosion took place on the ridges been built up at the foot of the ridge (fig. 3), when periglacial conditions prevailed during each fan matching a-solifluction valley system parts of the Weichselian (fig. 2). The sparse (fig. 4). During part of the Late Pleniglacial vegetation cover during the sub-arctic and arc­ very severe and dry climatic conditions tic phases of this stage, together with the abun­ prevailed, presumably a polar desert; these dant snow meltwater and the strong terrain conditions are documented by a deflation level gradient on the slopes of the ridges, led characterized by an aeolized pebble and stone to the formation of solifluction valleys (figs. 4 layer. In the.lower areas this band is covered by and 5). According to the geomorphological coversand. On the Ootmarsum ridge aeolized 38 M.W. VAN DEN BERG DEN OTTER & C. N s 497 Fig. 6. Cross section through the Mosbeek valley near the watermill of Bels. Holocene (anthropogenic) accu­ I. 494 mulation lying on top of the Weichselian basal gravel; 2. Weichselian solifluction head; ice-pushed strata. 3. gravel and boulders, often reshaped into ventifacts, are found in the topsoil. From the detailed facies analyses of the periglacial sediments carried out by Ruegg tz2Z22l,rrr:oll:-:-:-:-:-:-:14�sl = 1.1 _-=-"�17 (1981, 1983), it appears that after the forma­ c=J2 tion of the aeolized stone layer (called: Fig. 4. Generalized distribution of different lithologies Beuningen Gravel Bed) mainly wet-aeolian outcropping on part of the Ootmarsum push moraine conditions prevailed together with permafrost ridge. Tertiary strata: heavy clay; Tertiary strata: I. 2. other lithology; basal till of the Saalian ice sheet; 4. (Maarleveld, 1976). Consequently on the 3. Middle Pleistocene coarsegrained sands; 5. Weichselian flanks of the ice-pushed ridges sheet-like valley bottom deposits; quarry; 7. Weichselian solifluc­ 6. movement of the top layer could still take tion fans. place till the end of the Pleniglacial. During the Late Weichselian the vegetation became denser (fig. 2); permafrost conditions did not occur any more and the existing surface-relief became fossilized. The drainage of the water surplus concentrated into a channel-bound system of small brooks; the effective erosion caused by t.hese brooks is very small, but it occurs. c) This erosion is evident from the third group of processes: the human interference with this drainage system. Where man has -­ �. built dams te create water reservoirs to keep I water mills going, sediment accumulated in · I 70 " , .... _/ front of these dams (fig. 6) and the reservoirs , I , '. became silted up. These ponds have frequently \ , ':/l been dug out, to retain their water storage 'n capacity. Any archeological finds within these � Ikm o 70 L-_--'-__-' , spots are possibly strongly disturbed by human activity so geological data can hardly I----} E:J2�3�' EJs r:::zJ6 [SS]7 support the archeological interpretations. Fig. 5. Part of the Ootmarsum ice-pushed ridge showing the western flank with different drainage areas near Mander. water divide; direction of solifluction mass­ I. 2. 3. INTERNAL STRUCTURE OF THE movement; contourline in metres above N.A.P. 3. 00TMARSUM RIDGE (D.O.L.); 4. a'fcheological site with Middle Palaeolithic finds; 5. Mosbeek; 6. western boundary of outcropping ice-pushed strata; 7. outcropping Saalian outwash fan, In eastern Twente only clay-rich Tertiary partly covered by a Weichselian solifluction head. sediments are incorporated into the ice- Geology and morphology of a part of the Ootmarsum ridge 39 pushed ridges. These Tertiary sediments are The first have a high infiltration capacity, the marine deposits which are characterized inter latter are very difficult to erode by running alia by the presence of glauconite, a green clay water. mineral that can turn brown when it becomes Richter et al. (1951) have drawn attention oxidized. It stains the sediments green, yellow to the conspicuous ridges with slope angles up or brown. to 15° which are determined by outcrops of Before being pushed up by the land ice materials rich in gravelly sands, that belong to sheets, the Tertiary layers were covered in the Middle Pleistocene fluvial deposits. their undisturbed position by fluviatile sands The ice-pushed Tertiary deposits can be of Middle Pleistocene age. The latter are divided roughly into two lithological units: medium fine to coarse sands rich in quartz one group consists of heavy clays (±60% and mineralogically very poor in contrast to < 2j..tm); the other group contains all the the glauconitic sediments.