The Karurusu Clay Bed (13m in thickness) is deposited following the Noboribetsu Pumice Flow Deposit, from Kuttara Volcano, The 14C age of the Bed is older than 35, 500y.B.P. and is inferred to be a little older than that of the Shikotsu Pumice Flow Deposit (ca. 32, 000y.B.P.).
The paleoclimatic condition deduced from the pollen assemblage, such as Betula, Picea, Corylus, Alnus and other several broad-leaved trees suggests a little cooler than present. Clay mineral compositions of the Bed are montmorillonite, randomly mixed-layer mineral of montmorillonite and illite, kaolinite, pyrophyllite and degrated illite. Gypsum is also found.
