
ee 
346 STRUCTURAL AND FIELD GEOLOGY 
rubbly base of the clay. The latter thus becomes softened 
and slippery, and from time to time yields, and the mass 
creeps downwards. The boulder-clay, under such conditions, 
is ina state of unstable equilibrium, the risk of slipping increas- 
ing with the slope of the surface on which the clay reclines, 

