190 THE ICE AGE IN CANADA. 



The depressions between these ridges are occupied with 

 Pleistocene deposits, not so regular and uniform in 

 their arrangement as the corresponding beds in the great 

 plains higher up the St. Lawrence, but still presenting a 

 more or less definite order of succession. The oldest 

 member of the deposit is a tough boulder-clay, its cement 

 formed of gray or reddish mud derived from the waste of 

 the shales of the Quebec group, and the stones and 

 boulders with which it is filled partly derived from the 

 harder members of that group, and partly from the 

 Laurentian hills on the opposite or northern side of the 

 river, here more than twenty miles distant. The thick- 

 ness of this boulder-clay is, no doubt, very variable, but 

 does not appear to be so great as farther to the eastward. 



Above the boulder-clay is a tough clay with fewer 

 stones, and above this a more sandy boulder-clay, con- 

 taining numerous boulders, overlaid by several feet of 

 stratified sandy clay without boulders ; while on the sides 

 of the ridges, and at some places near the present shore, 

 there are beds and terraces of sand and gravel, constituting 

 old shingle beaches apparently much more recent than 

 the other deposits. 



All these deposits are more or less fossiliferous. The 

 lower boulder-clay contains large and fine specimens of 

 Leda glacialis and other deep-water and mud-dwelling 

 shells, with the valves attached. The upper clay is 

 remarkably rich in shells of numerous species; and its 

 stones are covered with Polyzoa and great Acorn-shells 

 (Balamts Hameri), sometimes two inches in diameter and 

 three inches high. The stratified gravel holds a few 

 littoral and sub-littoral shells, which also occur in some 

 places in the more recent gravel. On the surface of some 

 of the terraces are considerable deposits of large shells 



