Budowa geologiczna i rozwój doliny potoku Brusznik

Abstract

GEOLOGICAL STRUCTURE AND DEVELOPMENT OF THE BRUSZNIK STREAM VALLEYSummaryThe valley of the stream Brusznik is situated within the south-western part of the Izera Highland. It begins in the vicinity of Świecie then runs in a NNW direction to Leśna, to join the valley of the Kwisa river (Figs. 1 and 2). The valley of the stream here considered is, almost along its entire length, situated within the hilly area. Between Świecie and Leśna, along the sector of about 5 km, the valley is symmetrical, characterized by a flat bed (100–200 m).Geological examinations demonstrate that the Izera gnisses are the oldest rocks that build the river valley. The gneisses are overlain with weathered formations covered with a layer of gravels, sands and silts, several metres in thickness. On the left side of the valley (Fig. 1, bore holes H-13 and H-32) the bore holes entered Tertiary deposits (silts, sands, clays), According to palynological examinations (Fig. 3), the clay deposits originate from the Neogene-Palaeogene boundary (I. Grabowska, 1970). In the vicinity of the valley and within the valley area (bore hole K-25A and K-26) basalts and basaltic tuffs have been encountered. These formations are younger than the clay deposits (Upper Miocene, Pliocene). The Quaternary formations found in the vicinity of the valley are represented by fluvioglacial sands and gravels, arenaceous boulder clays, talus loams. Within the valley there are found gravels and sands of river accumulation, and silts with mica.The development of the valley began in the Oligocene. At that time, within the Izera Highland local intermontane basins were formed due to the tectonic movements of the Pyreneean phase. Within one of these basins the Brusznik stream does model its valley. At the turn of the Upper Oligocene and the Lower Miocene, the stream valley changed into a local sedimentary basin due to the activity of the Savian phase.At that time series of clay-arenaceous deposits were laid down. In the Upper Miocene time the tectonic activity of the Attican phase interrupted the sedimentation and, in consequence of this, these deposits underwent erosion. The Brusznik stream easily cut the clay-arenaceous deposits and reached the basement.At the Miocene and Pliocene times volcanic activity bagan. As a result of this activity three basaltic covers, separated with 20 m thick layers of loams and tuffs, were formed. In the Upper Pliocene the valley of the Brusznik stream became, due to the erosional processes, large and deep, and in this state it persisted till the glacial epoch. In the older and younger Pleistocene the valley was filled in with the gravel-arenaceous deposits (high infilling of the valley). At the Eemian interglacial time the Brusznik stream once again cut into the valley deposits, thus making a terrace 38 m above the stream level. In the Younger Pleistocene, during the dune-making phases, fine silts were laid down within the stream valley. These deposits end the sedimentation in the valley. At present, they are eroded, along with gravels, by the Brusznik stream.Brusznik is a type mountain stream. Along with its tributaries it drains an area of about 15 km2. Its mean runoff amounts to more than 15 1/sec/km2. During the freshets the runoff inreases and amounts to several hundred 1/sec/km2. The altitude difference between its spring and mouth is as much as 180 m, its length being equal to 8 km. Under the present-day morphological conditions the Brusznik stream intensely erodes the valley deposits mainly during the increasing atmospheric precipitations in the spring-summer period.