254 JOHN F. NEWSOM 



holes at the eastern edge are rarely more than 20 feet deep 

 and 200 yards across, while in the western part they are some- 

 times miles in length and from 50 to 200 feet deep, forming 

 valleys, similar in every respect to ordinary valleys of erosion, 

 except that they have no surface outlets for their drainage. The 

 increase in the size of the sink holes is, of course, due to the 

 greater thickness of the underlying limestone. 



The western edge of the Mitchell limestone is found just 

 west of the second principal meridian, where, overlying it, is the 

 lower Kaskaskia limestone, a hard, close-grained, resisting bed, 

 which is in turn overlain by a series of limestones and sand- 

 stones. The effect of these beds upon the topography is quite 

 noticeable. The hills rise higher and higher to the west, until on 

 the western edge of Lawrence county, where they are capped 

 by the highest beds of the Kaskaskia group, they are from 100 

 to 250 feet above the valleys. Many of these valleys are only 

 large sink holes, and have no surface outlets. 



The Mansfield sandstojie, or zvestem plateau. — Overlying the 

 Kaskaskia beds is the Millstone grit (Mansfield sandstone of 

 Hopkins) or the sandstone that forms the lowest member of the 

 Upper Carboniferous. This sandstone controls the topography 

 in the region in which it forms the surface rock, and is, in the 

 main, responsible for the high hills of Martin county. It has a 

 gentle westward dip and owing to this fact the highest hills of 

 the region are found near its eastern limit. The hills decrease 

 in height with the dip of the rocks to the west. The resulting 

 topography is essentially that of a thoroughly dissected plateau. 



The western lowland. — The Mansfield sandstone is finally lost 

 to view at the western edge of Martin county, a short distance 

 east of Loogootee. West of this point the country is level, or 

 very gently rolling. There is here an abrupt transition from the 

 rugged hills capped with sandstone, in Martin county to the 

 much lower, level country underlain by coal-bearing shales and 

 sandstones at the east side of Washington county. 



Overlying the Mansfield sandstone, and extending from its 

 upper limit to the west line of the state, the country is underlain 



