Abstract
River terraces are usually composed of those in various scales even in a valley. The subjects of the studies of river terrace made to date is mainly broad and dominant terraces, but small terraces is also important to interpret the developmental process of river terrace.
Not only broad terraces but also small terraces situated between Yamagata and Nagai Basin along the Mogami river were surveyed, and findings are summarized as follows;
1) River terraces are grouped into five (I-V) based on the zonal pattern in vertical distribution of terraces (Fig. 2). Formation of the tonal pattern of terraces relates to that of broad terrace surfaces such as the upper IV, which results in loss of the terraces such as III on the other hand.
2) Most river terraces are Strath terraces with gravel beds of several meters in thickness, but under formation of terraces III, Filltop terraces were formed at an altitude of 220m, which attribute to the effect of dam up by a landslide.
3) The paired terraces of the upper IV were aged about 30, 000 year B. P. and terraces of the lower IV are comosed of small terraces. Therefore, it is deduced that some change in factor with regard to the formation of river terraces occurred after 30, 000year B. P. The paired terraces distribute widely in the rivers running in the mountainous district, such as Ou Backbone Ranges, Dewa Hills and Kitakami mountains.
