DRIFT DEPOSITS OF ILLINOIS. 9 



nhioensis and several sp <]> of rodents, some of which belong 

 To extinct species. These discoveries seem to fix the age of the 

 loess ;is not later than The close of the Pliocene. 



The smaller teeth are usually found attached to, or embedded 

 in. the calcareous nodules that abound in the loess, which are 

 known under the popular name of "petrified potatoes." Locally 

 these nodules assume fantastic forms, similar to the common 

 "clay stones" That abound in the stratified clays of The Con- 

 necticut valley, and if they were of the same color, could not 

 be readily distinguished from the New England specimens. 



Fourth The fourth division of the drift of Illinois comprises the 

 boulder clays. These are usually yellowish at the top and bluish 

 gray below, and contain numerous boulders mostly of small size, 

 that are partly derived from foreign material, and partly from 

 the bed-rock of the adjacent region. This division of the for- 

 mation ranges in thickness from twenty, to more than a hun- 

 dred feet, and the embedded boulders vary in size from a few 

 inches, to two feet or more. Some of these transported ma- 

 are angular, as though they were embedded in shore ice, and 

 then transported by water currents to the spot where they 

 were finally dropped, while most of those of foreign material 

 are rounded and sometimes striated, as though brought in vio- 

 lent contact with angular fragments of still harder material. 



Trunks of large trees are sometimes met with in sinking wells 

 through the boulder clay, but no animal remains have been 

 authentically reported from it, to my knowledge, in this State, 

 except the Cretaceous fossils illustrated on a preceding 

 page. The boulder clay is frequently underlaid by a 

 black, peaty soil, varying jn thickness from two to thirteen 

 feet, filled in many places with twigs, branches, occasionally 

 whole trunks of trees, the wood in many cases being in a good 

 state of preservation. 



Fifth The fifth division comprises the ancient soil above men- 

 tioned, and the underlying stratified clays and sands, which lo- 

 cally includes a second soil, similar in character to that imme- 

 diately below the boulder clays. This division fills the ancient 

 valleys formed by erosive agencies during the earlier portion 

 of the drift period, and consequently is somewhat local in its 

 development. 



