546 CENOZOIC ERA— AGE OF MAMMALS. 



Quaternary Period in Eastern North America. 



I. Glacial Epoch. 



The Materials — Drift.— Strewed all over the northern part of 

 North America, over hill and dale, over mountain and valley, covering 

 alike, nearly all the country rock, Archaean, Palaeozoic, Mesozoic, and 

 Tertiary, to a depth of 30 to 300 feet, and thus largely concealing 

 them from view, is found a peculiar surface soil or deposit. It con- 

 sists of a heterogeneous mixture of clay, sand, gravel, pebbles, sub- 

 angular stones of all sizes, unsorted, unstratified, unfossiliferous. The 

 lowest part, lying in immediate contact with the subjacent country 

 rock, is often a stiff clay inclosing subangular stones — i. e., rock-frag- 

 ments with the corners and edges rubbed off. This we will call the 

 " Stony clay " or " Bowlder clay" It is precisely like the ground- 

 moraine of a glacier (p. 55). Over this is often found in places a looser 

 material with angular stones, like the top moraine of glaciers. Lying 

 on the surface of this drift-soil are found many bowlders of all sizes, 

 often of huge dimensions, sometimes even 100 tons or more. The im- 

 bedded subangular stones are usually marked with parallel scratches 

 (Fig. 923), and the large surface-bowlders are usually angular and un- 



Fig. 923.— Subangular Stone (after Geikie). 



scratched. The depth of this material is greatest in the valleys and 

 least on hill and mountain tops. 



It is difficult, nay, impossible, to give a description of this peculiar 

 deposit, which will apply in all cases. Sometimes scattered about ir- 

 regularly through the unstratified mass are portions which are roughly 

 and irregularly stratified, the laminae being often contorted in the 

 most fantastic way (Figs. 924-926). Sometimes the true stony clay is 



