A STUDY OF THE GEOMORPHOLOGY OF TOTTORI ALLUVIAL PLAIN

Abstract

The Tottori alluvial plain, which comprises the northeastern of Tottori prefecture, consists of the alluvial fans, the flood plains, the deltas and the coastal sand dunes. The microtopographical distribution of the alluvial plain is shown in Fig. l. The writer conducted an investigation of geomorphic surfaces and their deposits in order to understand the geomorphological history of this area. The results may be summerized as follows : l . The basement surface of the alluvial deposits is mapped in Fig. 3. The Kokubunji dissected fan can be traced to the surface of the basement of alluvial deposits and this dissection of Kokubunji fan was caused by regression after the alluvial maximum transgression. II. The process of changes of sedimentation of the alluvial deposits may be summed up as follows : I) In the maximum phase of the Wurm Ice Age, the trenchment of rivers on the surface of the buried river terrace was made. 2) Regression took place by 40meters below the present sea level. In this stage, the lower sand deposit was formed. 3) Rapid transgression took place, then the middle clay was deposited. 4) Retardation of transgression was seen by about 20 meters below the present sea level. Deposition of sand and gravel with clay was made, which is now seen in the middle of the geological profile. 5) In the process of the sequent rapid retrogression, the sea level witnessed a standstill, when the clay layer which is seen in the middle of the geological profile, was formed. 6) The sequent regression of small scale took place and then the upper layer of sand and gravel was formed, which comprises the present geomorphic surface.