SLOPE DEVELOPMENT OF TERRACE CLIFFS THROUGH GEOLOGIC TIME

Abstract

A study of slope development was undertaken on the cliffs of datable gravel river terraces in part of the Tokachi Plain of Hokkaido Island. The ages of the river terraces were estimated mainly by C-14 dating of overlying tephras with pumice-fall marker beds, and also by the sequence of terrace development relative to eustatic sea-level change. All the sampled slopes in unconsolidated gravels faced South but they differed in height and in the period of time since they had been abandoned by lateral river erosion. Twenty-nine slopes were measured, representing six different periods of time:<1, 2, 4, 6, and 10±104 years. They all show smoothed profiles consisting of three segments : an upper convexity, a straight segment and a basal concavity. Several indices of the slope characteristics of each profile were calculated and relations among them were shown graghically with the following results: 1. The percentage of the upper convex segment increases only slightly with time. The percentage of the straight segment varies inversely as that of the basal cocave segment. 2. The maximum slope of the straight segment of higher cliff's is independent of cliff height. It decreases with time rapidly at first, slowing down progressively. The maximum slope of lower cliffs varies with the height of the cliff, and also decreases with time. In the latter case, the upper covexity joins directly to the basal concavity formed mainly by deposition. Even in the former case, lateral river erosion would be indispensable to maintain a constant slope of the straight segment through time. In numerical terms, the maximum slope of cliffs higher than 30 metres is about 45° in the period shorter than 1±104 years and decreases to 30° by 6±104 years. The maximum slope of cliffs 20 meteres high diminishes from 40° to 27°, 22°, and 18° for ages of<1, 2, 4, and 6±104 years respectively whilst, for the same ages, the maximum slope of cliffs 10 metres high diminishes from 35° to 20°, 11°, and 10°. 3. The upper convexity is independent of cliff height, but is dependent on the maximum slope of the straight segment. The upper convexity of slopes younger than 1±104 years is very strong and sharp, but that of all slopes older than 2±104 years is weak and smooth. Since the upper convex segment is covered with undisturbed Holocene tephra layers, it is inferred that it is a fossil feature, probably formed under the periglacial climate of the last glacial period, and due to soil creep rather than slope wash. 4. Process on the straight segment is not so affected by climatic change as is on the convex segment. The slope of the straight segment has decreased continuously in recent 10±104 years.