Abstract
In the granitic area of the Abukuma Mountains, the writer found “Blockströme (boulder fields) ” and “Felsburgen” in several places. In these places, Felsburgen are usually located at the upper part of the mountain slope and Blockströme are found in the valley flat at the foot of the mountain slope. A Blockstrom at the foot of the Kôdaishiyama (altitude of 500-600 meters), the largest one in these places, is 500 meters long, 10-30 meters wide with the depth of 2-3 meters at least. The slope of this Blockstrom is 5-6 degrees on the average. Blocks in the Blockstrom are generally “Wollsackformen” of 1-3 meters in diameter, but a considerable portion of these blocks is broken with the sharp fracture planes.
In all Blockströme in this region, the present movement is unrecognizable and the running water at the bottom of Blockströme has a tendency to wash away interstitial soil material, resulting in the barren aspect.
It is thought these blocks in Blockströme were transported by means of mass movement from Felsburgen in a recent geological time.
As to the development of Blockströme in this region, in short, three stages are recognized : the first stage is a period of extensive sub-surface rock weathering whose pattern is controlled by structural conditions : the second stage is a period of exhumation by removal of the fine-grained products of rock decay (the appearance of Felsburgen and core stones) and transport of blocks by means of mass movement : the third stage is a period in which interstitial soil material was washed away.
