Abstract
From the results of a field and laboratory investigation, at the upper part of a ground water level, the non-silicified mud stone of Izumi group (Cretaceous period) shows a rapid weathering by the cyclic variation of a pore water content. So the banking and the talus deposit with this mud stone fragment are rapidly decomposed during short term and some-times are destroyed by a land slip.
The main reasons for this property of the mud stone are the followin three;
(1) this mud stone is not silicfied,
(2) this mud stone is consisted of the expansive mixed layer clay minerals and has a high swelling potential,
(3) this mud stone which deposited in the sea environment holds many soluble salts in rock pores.
At the southern foot of Izumi mountain system, there are many land slides which are mostly ascribed to such physico-chemical property of the mud stone.
