176 



til] thus varies with the character of the underlying rocks. The locally 

 formed pebbles and rock fragroeuts are chiefly sandstone, but numerous 

 foreign rocks and boulders of large size are occasionally found near the 

 limit of the drift. Tlie region presents^ a fairly even topography. lu 

 places, knolls and ridges with undulating surfaces occur, but in no place 

 do they become of great height. 



Striae are found in several places. They occur in Sullivan, Vigo, Clay, 

 Greene and Owen counties. The markings are chiefly upon sandstone ex- 

 posure. The drainage of the area covered by the Illinoian Invasion was in 

 many respects greatly modified. In attempting to work out the history 

 of an area whose drainage has been assisted by the invasion of the ice 

 sheet, the lite resolves itself into four fundamental parts. First. What 

 were the topographic characteristics of the area during the preglacial 

 history? Second. What changes took place during the glacial history? 

 Third. What has hapi>ened since the disappearance of the ice sheet; its 

 post glacial history? Fourth. What was the effect produced on the 

 uuglaciated parts of this area? The drainage is discussed to some ex- 

 tent under the heading of "Rivers and Lakes." 



I/f>c.s's Dcpof<iti)if/ Stage. — The lowan Drift. — rrior to the invasion by 

 the Illinoian ice Icibe there was a marked interval of deglaciation and a 

 similar interval occurred at tlie close of the Illiiinian jieridd. These inter- 

 vals were marked l)y leaching and oxidation of the drift, the accumulation 

 of i)eat and soil, and the jtrocesses of erosion. The intevglacial interval 

 following the Illinoian invasion is known as tlic Sangamon Stage. 



The surface of the Illinoian drift outside the limits of the Wisconsin 

 drift is covered with a fine grained yellowish silt or loam, to which the 

 term loess has been apijlitnl. Loess is a deposit wliidi. like sand or gravel, 

 may lie laid down whenever conditions are favorable, but since the great 

 bulk api>ears to have been deposited at a definite stage in the glacial pe- 

 riod, the time of deposition may be referred to as the Loess Stage. This 

 loess may be of different ages, but since the materials contained are such 

 as occur in glacial drift it juust have been derived from the drift. The 

 source is supposed to be from the lowan drift, and the distribution due 

 to the combined action of wind and water. The loess of Indiana varies 

 from a fine silt of a loose floury texture to compact masses, held firmly 

 by a calcareous cement. In some places small pebbles are found imbedded, 

 also fossil remains of fresh water mollnsks. and some insects and bones 



