176 



till thus viiries with the ohiirad* r of the umlerlyins rocks. The locally 

 foi'ined pebbles and rock fragments .ire chietly sandstone, but numerous 

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

 limit of the drift. The region presents a fairly even topography. In 

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

 do they become of great height. 



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

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

 posure. Tho 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 life resolves itself into four fundamental parts. First. What 

 were the topograjihic characteristics of the area during the preglaeial 

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

 Third. What has happened since the disappearance of the ice sheet; its 

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

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

 tent under the lieadhig of "Rivers and Lakes." 



Loess Depositing Stage.— The loicaii Diift.^rriov to the invasion by 

 the Illiiidian ice lobe there was a marked interval of deglaciation and a 

 similar interval occurred at the close of the Illinoian i)eri(i(l. These inter- 

 vals were marked by leaching and oxidation of tlie drift, tlu' accumulation 

 of peat and soil, and the iirocesses of erosion. The intorglacial interval 

 following the Illinoian invasion is known as ilic Sang.-iinnn Stage. 



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

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

 term loess has been api)iie<l. Loess is a dciiosit which, like sand or gravel, 

 may bo laid down wlicnevcr conditions are favorable, but since the great 

 bulk appears to have been depfisited at a definite stage in the glacial pe- 

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

 loess may be of different ages, but since the materials containe<l are such 

 as occur in glacial drift it must have l)cen 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 w.itcr. 'i'lic loess of Indiana va.ries 

 from a fine silt of a Ions;- llonry levlui-c to coniit.Mct masses, held lirndy 

 by ft calcareous cement. In some iilaces small iPcMiles are found imbedded, 

 .ilso fossil remains of Ircsb walci- niollusks. and some insects and bones 



