522 REPORT otf STATE GEOLOGIST. 



PHYSIOGRAPHIC FEATURES. 



"Many import;! nt land forms are wanting in Indiana. There are no 

 mountains, no valleys formed by upheaval or subsidence, no volcanoes 

 or volcanic rocks except foreign fragments brought by the ice sheet, 

 no features due to disturbance of the earth crust except the rock foun- 

 dations of the State itself. 



"Plains. As already indicated, the greater part of Indiana is a plain 

 of accumulation; the surface of a sheet of glacial drift which varies in 

 thickness from a few feet to 500 feet or more. The average thickness 

 is more than 100 feet. It consists chiefly of a mass of clay containing 

 more or less gravel and bouldersthe till or boulder clay of the geol- 

 ogists. This is locally varied by heaps, ridges, sheets and pockets of 

 sand and gravel, and in the southern part of the State is overlain by 

 a peculiar fine silt called loess. The boulder clay is the grist of the 

 glacial mill, and is composed of a very intimate and heterogeneous 

 mixture of native and foreign materials, containing fragments of 

 almost every known mineral and rock. The large fragments, or 

 boulders, are widely distributed, and of every size up to 30 feet in di- 

 ameter. They are nearly all igneous or metamorphic in character and 

 can be traced back to their origin in the Canadian highlands north of 

 the Great Lakes. 



"The driftless area is a plain of degradation, formed by the removal 

 of the original rock surface to an unknown depth, and now reprre- 

 scnied by the summits of the flat and even-topped divides^ ridges and 

 hills. 



"Hills. On the northern plain occur numerous hills of accumula- 

 tion forming the great morainic belts, the result of excessive dumping 

 and heaping up of drift along the margins and between the lobes of 

 the melting ice-sheet. The most impressive examples are found in 

 Steuben, Lagrange, Noble and Kosciusko counties, where they attain 

 a height of 200 feet or more, and are as steep and sharp as the mate- 

 rials will lie. Their peculiar forms and tumultuous^arrangement give 

 a striking and picturesque character to the landscape. 



"The Ohio Slope is studded all over with hills of degradation blocks 

 and fragments of the original plain left by the cutting out of the 

 valleys between them. Some are broad and flat-topped, some narrow, 

 crooked and level-crested, some sharp or rounded, isolated knobs or 

 lnilics. These are very conspicuous in the counties of Greene, Da- 

 viess, Martin, Crawford, Orange, Washington and Jackson, W attain 

 their greatest development in Floyd, Clark and Scott, where the 

 Silver Hills and Guinea Hills rise to 400 and 500 feet above the valley 



