Zastosowanie metody elektooporowej do badania czwartorzędowej rzeźby kopalnej na przykładzie Doliny Dolnej Skrwy

In the valley of the Lower Skrwa, below Cieślin (Fig. 1, 2, 3) the method of electric resistance has been used in the reconstruction of the Quaternary fossil relief. On the basis of several hundred geoelectric soundings carried out in Schlumberger’s symmetrical system (Fig. 4) three groups of deposits were distinguished in the investigated area: the high-resistance sandy group and the medium-resistance clayey group represent the Quaternary, and the low-resistance loamy group - the Pliocene. The analysis of the horizontal and vertical lability of those groups of resistance, based on the configuration map of the substratum surface of Quaternary deposits and on the map of the reach of interglacial river sands about 20-60 m thick (Fig. 5), as well as on eleven geoelectric cross sections of Quaternary deposits (Fig. 6, 7), has allowed an exact characterization of the various deformations occurring in the sub-Quaternary surface, as well as a thorough study of the system of the river valleys in the Mazovian and Eems interglacial periods. Against the background of the principal deformations of the sub-Quaternary surface in the Lower Skrwa Valley, i.e. of the Płock elevation and the Mochowo depression (Fig. 2) second-rate deformations can be distinctly observed: they form three hypsometric steps: a high step (np to over 100 m above sea level), a medium step (0-80 m above sea level) and a low step (from 20 m below sea level to over 0 m above sea level); there also occur still smaller, third-rate deformations, i.e. long, winding furrows and oval or round hollows, pot-holes and hummocks (Fig. 8). In the author's opinion the origin of the above-named fossil forms of the sub-Quaternary surface is connected with the glacitectonic activity of the ice-sheet, with the erosion of interglacial rivers as well as with the pressure of thick packs of Quaternary deposits (mainly boulder clay) on plastic Pliocene clays (isostatic deformations). On the base of the geoelectric cross-sections of Quaternary deposits the evolution of the fossil valley network between Cieślin and Brudzeń Duży (Fig. 9) has been investigated. It has been stated that in the Mazovian interglacial period there existed an important hydrographic junction, i.e. several large and deep river valleys met here. Instead, Eems river valleys, much smaller and shallower and thus less legible in cross-sections, partly duplicated the run of valleys of the Mazovian (Kromerian ?) interglacial period. A similar situation occurs in the present, deeply cut valley of the Lower Skrwa and in some of its side valleys, which proves - in the author’s opinion - the repeatability (conservatism) of valleys of different age in the investigated fragment of the Płock Upland. In comparing the method of electric resistance with the classical interpretation of geological borings it should be noted that, unlike the punctual minuteness of detail afforded by the interpretation of borings, the method of electric resistance allows to get a spatial (though less accurate) image of the fossil relief, especially of large forms built of deposits of little lithological variability.