Abstract
The writer has conducted detailed field surveys on landforms and structures of valley head areas in hilly lands of the Nishitanaka region, the vicinity of Sendai, in order to clarify the process of valley development.
The valleys in this area are composed of three topographic units, i. e. valley head, Vshaped valley, and V-shaped valley with flat bottom, from upstream to downstream. The valley head is classified into three types, namely, large-trough type (large shallow valley head with concave profile) (Figs. 3, 4-a, b), spoon-shaped type (Figs. 3, 4-c), and small-trough type (Fig. 3).
The built-up periods of these valley head types can be inferred from the correlations with terrace surfaces.
The terrace of Nishitanaka can be classified into three surfaces of I (hill top surface), II, and f (fill top terrace) in descending order of elevators (Fig. 1).
Large-trough type valley head is considered to have been formed between the formative periods of the terrace I and 1f, about 30, 000-60, 000 y. B. P. Under wet and cool climate toward the Wurm glacial stage, flow type landslides (Varnes, 1958) with volumes of on the order of 104 cubic meters took place on bedrocks and terrace gravels which had been weathered during the preceding interglacial stage. As a result of occurrence of flow type landslides, large-trough type valley heads were scraped out and landslide debris filled the floor of V-shaped valleys and formed gentle slopes on the terrace I surface (Fig. 6).
Spoon-shaped type valley heads have been extended toward upstreams by headward erosions through the late Wurm glacial stage (30, 000 y. B. P.) to the present. The headward erosion is resulted from slide-type failures on the order of 103 cubic meters in volume, which occurred along joints in bedrocks and pipe networks (Whipkey, 1965) in soils (Fig. 7).
Small-trough type valley head occurred on valley-side slopes head hollows of largetrough type valley head as results of non-cyclic landslides.
Development of valley heads has been generally controlled by the change of local base-level and the topographic feature of each valley head types has been characterized by climatic changes.
