The basal topography of the latest Pleistocene to Holocene incised valley fills under the Arakawa Lowland, Kanto Plain, central Japan is revealed based on 2580 borehole log data. The base of the valley fills in each borehole log is identified based on soil properties and comparison with the data of all-core sampling surveys. These point data composed of longitude, latitude and elevation of valley fill base are interpolated to describe the basal topography by the inverse distance weighting method. The basal map shows a detailed topography of the paleovalley, including buried wave-cut benches and buried terrace planes. The gradient of the paleovalley floor is approximately 1/1000, which is almost the same as that of the neighboring Nakagawa lowland. In some places around the Omiya upland, it would be very difficult to differentiate the Holocene valley fills from Pleistocene strata with similar properties.
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