The Quaternary Research (Daiyonki-Kenkyu)

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The involution structure in the Tokachi area of Hokkaido develops in two clearly bounded layers (upper: loam; lower: pumice), and the deformation pattern is fairly regular, with wave length of less than 1.5m and amplitude of 0.3 to 1.0m. Bulk density and viscosity of loam and pumice beds during the deformation were estimated from laboratory measurements. The bulk density of saturated loam is larger than that of saturated pumice. Under the assumption that the deformation was caused by this reversed density stratification, the wave-length of involution can be calculated by RAMBERG'S model. The result of the calculation is about 1.8m, which well approximates the field data. The vertically asymmetrical form of the interface of loam and pumice beds can be explained by a kinematic viscosity contrast of the two layers, as implied by ANKETELL (1970).
When the driving force is only gravity, a reduction in the strength of the bed material is needed. Thawing of highly frozen ground, combined with poor drainage (flat topography over an impermeable bed), could offer the most suitable conditions for (over-) saturation and strength reduction of the fine-grained material. The involution structure shows that the continuity of deformation was interrupted by a cessation in strength reduction. Such short-term and extreme strength reduction might have resulted from such accidental events as liquefaction of saturated and cohesionless sediments. At least in this type of deformation, both RAMBERG'S and ANKETELL'S schemes are valid, although the possibility of secondary modifications of features by other periglacial actions such as frost heave should not be neglected.

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Holocene vegetation around Lake Hamana was reconstructed from fossil pollen assemblages in three boring core sediments obtained from the bottom of the lake (the deepest point of the main lake and its satellite, Lake Shonai) and the lakeshore (near the Town of Yuto-cho).
Three forest stages were recognized at each site, namely, warm-temperate broadleaved deciduous forest, lucidophyllous forest, and Pinus forest stages.
The development pattern of the lucidophyllous forest in the northern area was different from that in the southern area. In the northern area, which includes the catchment of the Miyakoda River, the major supply source for the main lake, lucidophyllous forest composed of Castanopsis and Quercus (Cyclobalanopsis) developed, and flourished from about 5, 000y.B.P. On the other hand, in the southern area around the Town of Yuto-cho and Lake Shonai, Castanopsis forest formed about 7, 500y.B.P. and Quercus (Cyclobalanopsis) began to expand from about 6, 000y.B.P.
These facts indicate that the southern area of Lake Hamana facing the Pacific Ocean must have been influenced by the ocean, especially the warm Kuroshio current, during the Holocene.

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