The Shimosueyoshi upland in and around Yokohama City is 30-50m high and borders the eastern part of the Tama hilly land. This upland consists of the Shimosueyoshi formation, deposited at the time of the Shimosueyoshi transgression which has been thought to be correlated with the last interglacial age, and its surface is covered with the eolian deposits, the Kanto volcanic ash bed. The original surface of this upland is, therefore, the marine terrace plain representing the high sea level of the Monastrian stage. This area has long been studied as one of the type localities in the Japanese Quaternary successions.
The detailed discussion, however, has not been made completely as to the morphological characteristics which are different from place to place. The purpose of this paper, therefore, is to investigate the following three problems in more detail: (1) the topography of the Shi-mosueyoshi terrace, especially the altitude distribution (Fig. 1) and the drainage pattern and density (Figs. 2 and 3), (2) the bedrock topography with respect to the facies and thickness of the Shimosueyoshi formation (Figs. 4, 5 and 6) and (3) the relation between the present topography and the bedrock topography according to the data shown in these figures. This summary describes particularly the results of the above-mentioned (2) and (3) which have not been fully discussed in the previous studies.
The bedrock topography underlying the Shimosueyoshi formation is classified into the fol-lowing three groups:
1) Group A is the very flat surface situated closely in the east side of the former shore-line with a height of 40-45m on the Shimosueyoshi terrace. This flat surface inclines gently to the east down to 30m high and is characterized by Lebenspur of the boring shell. Accord-ingly, the bedrock topography of Group A can be regarded as an abrasion platform which was formed at the time of the highest sea level of the Shimosueyoshi transgression. The Shimo-sueyoshi formation covering this abrasion platform is sandy in grain size and its thickness is less than 1 m. Consequently, the original surface of the Shimosueyoshi terrace in the Group A area may be inferred to represent the abrasion platform itself.
2) Group B is distributed in the east side of Group A. Its altitude is 25-10m above sea level inclining gradually to the east. This group is also regarded as an abrasion platform be-cause of its flatness and Lebenspur of the boring shell remaining on it. But it is different from Group A in the fact that the thickness of the Shimosueyoshi formation is several meters, considerably thicker than in the Group A area. Accordingly, it can be estimated that this abrasion platform was formed by the wave erosion during the time of high sea level when the Shimosueyoshi transgression did not reach its maximum. The advance of this transgression caused a gradual increase of the depth of water and turned the Group B area into the depositional one of the neritic sediments, accompanying the migration of the effective wave erosion toward the west to form the abrasion platform of the Group A area. The original surface of Group B area, therefore, can be concluded to be a depositional surface, rather than an abrasion platform itself.
3) Group C is observed along the main rivers and in the east side of the Group B area. Topography of this group is characterized by the high relief with various valley topographies which were formed by the subaerial erosion preceding the Shimosueyoshi transgression. Those valleys were drowned during this transgression, and then filled with the Shimosueyoshi forma-tion which showed local differences in its thickness and lithological composition in accordance with the initial relief of the bedrock topography.