Erosional low-relief surfaces on the mountains of the Oshima Peninsula are divided into two levels. The higher surface is called the Matsumae surface and the lower one is called the Kikonai surface in the Matsumae Peninsula of the southern part of the Oshima Peninsula. Coastal terraces spread below and around these erosional low-relief surfaces. The Esashi district is one of the areas where coastal terraces are typically developed in the Oshima Peninsula. The author discussed the physiographic development and the crustal movement of the Esashi district.
The coastal terraces in the Esashi district are classified into five surfaces as follows:
Esashi surface (Pleistocene); 340-140m a. s. l., 1500m wide.
Oyama surface (Pleistocene); 170-80m a. s. l., 2000m wide.
Ohma surface (Pleistocene); 80-40m a. s. l., 500m wide.
Jinya surface (Pleistocene); 45-20m a. s. l., 250m wide.
Kitamura surface (Holocene); 12-8m a. s. l., 100m wide.
The Esashi surface is presumed to have been formed by the transgression over the Kikonai surface which was the lower erosional low-relief surface.
The Oyama surface is the widest marine erosional surface in the Esashi district. This terrace was formed by the transgression over the Esashi surface which was gently sloping toward the sea.
The Ohma surface was formed by the transgression with larger than 20m rise of the sea level. The coast at that time, however, was a ria coast.
The Jinya surface was formed about 30, 000 years B.P. At that time, the rise of the sea level was larger than 7m. It is suggested that the minor transgression took place at the Würm interstade about 30, 000 years B.P.
The Kitamura surface was built by the Postglacial transgression with larger than 50m rise of the sea level. A part of the Kitamura surface is presumed to be a marine terrace formed by a minor transgression about 3, 000 years B.P.
From the height of the old shorelines of these terraces, the crustal movement of the tilting toward the north in the area of Palaeozoic sedimentary rocks, the warping in the area of Miocene sedimentary rocks in the Esashi Hill of the northern part of the Amano River and the tilting toward the south in the Kaminokuni Hill of the southern part of the Amano River are recognized. The crustal movement is presumed to become relatively active after the period between the formation of the Oyama terrace and that of the Ohma terrace.
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