DKIFT DEPOSITS OF ILLINOIS. 7 



the borders of the Mississippi valley, and southward until they 

 united with strata of the same age south of the Ohio river. 



After this brief view of the conditions that probably prevailed 

 during the earlier portion of the drift period, we may now pro- 

 eeril to note briefly the several subdivisions of the drift deposits. 

 Taking them up in the descending order, they are as follows: 



First we have the alluvial deposits of the river valleys, or 

 those below the ordinary flood plain of our large rivers. These 

 deposits consist mainly of sand and fine silicious sediment, held 

 in suspension by the water during periods of inundation, and 

 were deposited over the flood plain as the waters receded to 

 their ordinary level. 



Second We have the river terraces that were formed in a 

 similar manner to the river bottoms, but at a time when the 

 flood plain was from thirty to forty feet higher than at the 

 present time. Those terraces form eligible sites for man}' of our 

 river towns, among which we may mention Naples, Meredosia 

 and Havana, on the Illinois river, and Oquawka and New Bos- 

 ton on the Mississippi. The terrace ''epoch" is probably nearly 

 synchronous with that of the loess. 



Third Modified drift and loess. The term modified drift is 

 applied to beds of sand, gravel and clay that have been formed 

 from the older members of the series by the sifting process of 

 wave action or water currents. This deposit generally forms 

 the upper portion of the river bluffs, and often fills lateral val- 

 leys on the main water courses. 



The bluffs at Warsaw in Hancock county, presents an instruc- 

 tive section of modified drift and loess. These deposits are 

 there from forty-five to fifty feet in thickness, and were cut 

 through in grading Main street to the river landing, showing 

 the following section: 



Ft. In. 



Ash-colored marly clay 10 



Brown clay 10 



Brown sand stratified 8 



Blue sandy clay 2 G 



Fine gravel and clay 2 6 



Yellow sand 2 



Gravel and boulders 8 



Bluish clay exposed 5 



