QUATERNARY FORMATIONS — THE DRIFT. 199 



CHAPTER Y. 

 QUATEEN"AEY FOEMATIONS — THE DEIFT. 



The formations of this region consist of two great classes, the one 

 made up of indurated rock, the other of loose material in the form of 

 clay, sand, gravel and bowlders. The former had their origin in de- 

 posits made by the ancient ocean, which have become hardened to the 

 state in which we now find them. The others had their primary ori- 

 gin, as is now generally believed, in the action of ice in the form of a 

 glacier. Subsequently a large portion of this material was washed 

 out and redeposited or otherwise modified by the action of lakes and 

 rivers. That portion which remains in the condition in which it was 

 originally left by the glacier is known as unmodified drift, while that 

 which has been rearranged and redeposited by the subsequent action 

 of water is termed modified drift. Taken together, they constitute 

 the chief Quaternary formations. Since they occur at the surface and 

 are diverse from the bedded rocks below, in their nature and origin, 

 it will be most convenient to consider them independently. 



The primary drift, or that portion of the material which exists in 

 the condition in which it was left on the retreat of the glacier, will 

 be considered under the head of (1) moraine ridges, and (2) bowlder 

 clay or till. The secondary or modified drift consists of a series of 

 beach deposits of sand and gravel, and of lacustrine deposits of clay. 

 In addition to these there are many local deposits made by rimning 

 streams and small lakes that will not receive special consideration. 



Glacial Movernents. To clearly understand these drift formations, 

 it will be desirable to first study the character of the glacial move- 

 ments which gave rise to them. At the commencement of the great 

 cold period which led to the vast accumulations of ice which over- 

 spread this portion of' the continent, this part of Wisconsin had 

 already been long elevated above the surface of the ocean, and had 

 been subjected to very great erosion, by which extensive and deep val- 

 leys had been formed. There can be no doubt that the Green Bay and 

 Kock Eiver valleys were already in existence, though doubtless pre- 

 senting many features very different from those exhibited at the i res- 

 ent time. There is abundant evidence that some of the streams had 



