The Middle and Upper Pleistocene Development of the Lower Jizera Valley

Abstract

This paper contents a short characteristic of the geomorphological conditions and of the development of the valley of the lowest Jizera (= the right tributary of the river Elbe). On the base of the geomorphological analysis, of the detailed longitudinal section and of numerous cross sections two great accumulations of gravel and sand of the river terraces VI and VII were distinguished, belonging probably to Middle Pleistocene (Riss 1, Riss 2). The sediments of the VIth and the VIIth terraces, with the surfaces at a relative heigth of 20 and 13 m above the low level of the river, are developed in the extensive area of the confluence of the Jizera with the Elbe (the so called Jizera-delta) making to flat dejection cones resembling forms. Their origin is connected with an abrupt decrease of the stream gradient of the Jizera at the entrance into the valley of the middle Elbe (Labe). While the maximum thickness of the sediments of the VIth terrace (up to 25 m) being preserved under the major part of the surface of this level, the greatest thickness of gravel and sand of the VIIth terrace (21 m) is developed only above the relatively narrow (mostly fewer than 1000 m) so-called overhollowed furrow on the valley bottom. Besides the accumulation surface of the VIIth terrace (VIIa) there are five lower steps in the sediments of this terrace level, representing from the genetic point of view on the one hand the erosion terraces and on the other the imbedde ones. The Upper Cretaceous sediments (marlites) in the substratum of these lower terraces (VIIb-VIIf), regularly graduated between the surface of the terrace VIIa and the valley plain (the level VIIg), are not situated anywhere under the level of the base of the terrace VIIa. Besides the course of the base in the cross section for the genesis of the VIIth terrace there is a very important area close before the entrance of the Jizera into the Elbe valley plain. In this area on the 1 km long river bed no valley plain together with the terrace VIIf is developed and the river is epigenetically downcutting into the marlite elevation buried mostly under the river gravel and sand of both the Middle Pleistocene accumulations, here and there levelled down into the level of the surfaces of the higher steps of the VIIth terrace. Among the steps of the VIIth terrace the Würm-accumulation is probably hidden, established on other Bohemian rivers. It is also the imbedded terrace with a base altogether lying above the level of the rock substratum of the VIIth terrace (here and there also of the VIth terrace). The origin of the geomorphologically perfect stepland of the low terraces on the lowest Jizera was produced by a number of the complex active causes, from which besides the favourable geological structure and the stream gradient the horizontal and vertical oscillation of the Elbe was expressed, flowing approximately perpendicular to the direction of the course of the Jizera. The level of this river reacted on each change in the position of the base level of erosion of the main river.