THE IOWAN LOESS. 165 



From the time of the earliest recognition of the loess in the Mississippi 

 Basin the presence of fossils has been mentioned as a peculiarity of the 

 deposit. These fossils are present, not as ingredients of a mixture, like the 

 wood or other organic remains found in bowlder clay (whose life was inde- 

 pendent of the deposits in which they are embedded), but as a fauna repre- 

 senting the life of the region during the progress of the accumulation of 

 the deposit and dwelling upon it or in it. 



The loess fossils are found in greatest abundance along the immediate 

 borders of the main valleys where the loess attains its greatest thickness, 

 notably along the Mississippi, the Illinois, and the Wabash. They occur 

 from the top to the bottom of the deposit and bear clear evidence of having 

 lived during its deposition. Along the smaller valleys of the region they 

 are rarely found. On the interfluvial tracts but few fossils have been found 

 at a distance greater than 5 miles back from the borders of the main valleys. 

 The most notable exception is in the thickened loess border south of the 

 Green River Basin, where the loess is thought to have been deposited along 

 the Iowan ice front, though here they are far less numerous than along the 

 main valleys. Fossils are also distributed more widely along the Sangamon 

 than along other tributaries of the Illinois, being found as far back as the 

 vicinity of Springfield. 



The cause for the scarcity of fossils along the small streams and in the 

 interfluvial tracts can as yet scarcely be decided. It is not known whether 

 they have been destroyed there by subsequent leaching and weathering or 

 were never present. Professor Shimek, as indicated more fully below, has 

 found that in eastern Iowa the living representatives have about the same 

 distribution in relation to streams as those embedded in the loess, and he 

 infers that the loess fossils were never present in abundance at a great 

 distance from the main valleys. 



The most abundant and widely distributed fossils are mollusks, of 

 which terrestrial species predominate over aquatic. By far the most com- 

 mon mollusk is Succinea avara, a form which is now found in swampy 

 places as a rule, but which occasionally occurs in dry situations. Shimek 

 regards this as a strictly terrestrial rather than a semiaquatic form. The 

 tables given below serve to indicate the proportion of species of terrestrial 

 and aquatic forms. The shells of a few unios and other strictly fluviatile 



