南部 아이다호州의 花崗岩 tor地形 考察

Abstract

Around the Rock City in the southern part of the state of Idaho, for landscapes appear intensively on the upper plateau which is 2000m a.s.l. The parent rock of these tors is composed of post-Cretaceous plutonic granite. All the tor rocks are hornblende granite which is coarse-grained and light colored. These tor landscapes are divided into three clssification: large (13~15m, in height), medium (7~10m) and small-size tors(5~6m). The development of tor landscapes resulted from the erosional processes, joint patterns, and fracture structures of the respective parent rocks. The size and shape of tor depend upon the space and directions of its joints as well

:as regolith characteristics. The tor of Rock City is similar to No.2 which was shown in earlier discussions of this paper and if it is ,compared to Fig. 3 it belongs to 'C' and 'D' category. The exhumation of this tor landscape has been experienced and modified by subaerial denudation. The large tors, which have dome shape and curved joints, :are similar to collapsed bornhardts in a

sense. (Cunningham, 1971) The bed rocks were weathered deeply and removed by periglacial mass movement and formed low gradient plains. As a result, small creeks started to appear. The study area, which has risen several times and at some time fractured, had warm, humid climatic conditions earlier.

The study area has been experienced by periglaciation during Quaternary as well as at present. There are in addition numerous microforms such as relict duricrust, tafoni and pan-holes on both surfaces and sides of the tors. Still, there are some problems to be studied, concerning the age of tor landscapes, microforms and slope development. The problem with tors is that they are examples of convergent evolution, whereby quite different processes can produce the same end result. It may well be that individual tors can form different sets of processes and operating tors can form at different

times. As these tor phenomena appear on any climate areas from the point of climate geomorphology (Klimmamorphologie) it is important for me to say that within the azonal granite landforms system the tor study must be conducted by relating climatic and geologic factors.