DRAINAGE OF SOUTHERN INDIANA 167 
valleys have been cwt out entirely since glacial times, leaving the 
drift in remnants only upon the hilltops and uplands. 
Effect of the structure in the region thinly covered by the drift.— 
The thin mantle of drift that covers that portion of the state 
east of the driftless area lies on strata that dip gently to the 
southwest, and on an old surface whose general contour prior to 
the deposition of the drift was similar to that of the present 
time. Therefore, while the minor lines of drainage have been 
modified by the drift, the general south and southwest drainage 
of the country is such as would be logically developed in a 
country of such combinations of hard and soft southwestward 
dipping strata as southern Indiana possesses, and it is practically 
the same now as it was in preglacial times. 
The series of strata that control the topography and drainage.— 
There are in southern Indiana three thick series of shale beds, 
between which are groups of harder and more resisting lime- 
stones and sandstones. In going from east to west across the 
state these groups are as follows: (See the numbers on the 
cross sections shown on the accompanying drainage map, Plate 
I.) (1) the Hudson River shales, along the east side of the 
state; (2) the resisting Niagara limestone, and limestones at the 
base of the Devonian; (3) the New Albany and Knobstone 
shales, all soft and easily eroded beds; (4) the Knobstone sand- 
stones and overlying Carboniferous limestones, which are in turn 
overlain by the sandstones at the base of the Coal-measures ;? 
(5) the soft Coal-measures, shales, and sandstones of the west 
side of the state. 
Postglacial and preglacial topography—The softer groups of 
strata, viz., I, 3, and 5, form drainage areas (discussed below as, 
the eastern, central, and western drainage areas respectively ) 
that are more or less separate from each other in each case, 
while the harder groups, 2 and 4, form the highlands or water- 
sheds between those areas. 
* The Lower Carboniferous limestones are eroded more easily than the beds lying 
both east and west of them, but their denudation has not been so great as to form a 
separate drainage basin in the area underlain by them. 
