Geological Investigations on Boulder-Clay of E. Groningen

In this article the results of a study on boulder-clay in the neighbourhood of Winschoten (N.E. Netherlands) are communicated (Chapter I). The underlying sediments of the boulder-clay in this area consist of fine preglacial sands and black clay. In the nuclei of the many drumlins a strongly ice-pushed boulder-clay may be encountered (Chapter II). Palynological analysis showed the pollen content of the boulder-clay to be very small. In a few samples more pollen was found (Plates I and II), but in these cases there appeared to be an admixture of black clay, obviously picked up by the land-ice. This black clay (the so-called potklei or pottery clay) is very humic and resembles the Lauenburg clay from Germany, but is younger. Using pollen analysis only one would date this clay as Miocene or even older (Plates II and III). This is impossible however, for in borings in this area Pleistocene sediments underneath the potklei are encountered. The solution of the problem is that we are dealing bere with secondary pollen material, originating from the Miocene in N.W. Germany; this pollen was transported by rivers before the land-ice came (Chapter III). Granulometric analysis proved the boulder-clay of Winschoten to be the normal Dutch type. As far as we know this boulder-clay was deposited during the Saale glacial (Chapter IV). The erratics in two samples were carefully examined. To this purpose the erratics from 6 mm — 5 cm were counted (according- to the Madsenmethod 1897); the results were arranged in such way that a comparison with the countings from De Waard (1947) in the N.O. Polder could be made. Therefore the percentages of the various groups of erratics taken from the total content of erratics were compared with each other (Chapter V). Fig. 4 shows the countings. It will be seen that the number of crystalline erratics in the boulder-clay from Bovenburen is considerably smaller, the sandstone and quartzite content far greater than that found in the boulder-clay from the N.O. Polder. In the field too, this was striking. We might speak of a local association of erratics in the grey boulder-clay at Bovenburen. The analysis of the light fraction (Chapter VI) gave the following data: the composition of the samples, the roundness and dullness of the quartz grains correspond with the data from the normal grey boulder-clay (Table VI). This agrees with the fact that the microfossils mentioned in this article were only found in grey boulder-clay. A small admixture of red boulder-clay is possible however, on account of an occasional find of some brown bryozoa and ostracoda characteristic for the red boulder-clay. Moreover the identification of the bryozoa indicated that fine components of the boulder-clay we examined originated from an area (Denmark and S. Sweden) with Danian and Upper Senonian outcrops (Table V).