DRAINAGE OF SOUTHERN INDIANA 169 
some short streams that rise on the Niagara strata and flow east- 
ward into the Hudson River area; streams of this area are shown 
by dotted lines on the accompanying map, Plate VI; (2) the cen- 
tral area, covered by the strata between the base of the Niagara 
and the Knobstone sandstone; streams of this area are shown 
by solid lines on the accompanying map, Plate VI; (3) the west- 
ern area, covered by the rocks from the Knobstone sandstone to 
the top of the Coal-measures; streams of this area are shown 
by broken lines on the accompanying map, Plate VI. This last 
area includes the entire southwestern part of the state, and in 
the eastern part of it the streams have in many places cut down 
through the limestones and Knobstone sandstones, and into the 
Knobstone shales. These shales, however, have had no part in 
the formation of the western drainage area, which, while largely 
underlain by sandstones and limestones, has its eastern watershed 
along the Knobstone escarpment within a few miles of the lowest 
part of the central area. 
From each of the watersheds, viz., between the eastern and 
central, and the central and western areas, the streams that flow 
east across the dip of the strata are short and have steep gradi- 
ents, while those that flow west with the dip of the strata, are 
long and have low gradients. 
The larger streams of southern Indiana flow through filled 
valleys. The depth to which the valleys have been filled varies 
from a few feet to over one hundred feet. 
The different drainage areas will be taken up in their order, 
and the features of their drainage so far as these depend upon 
the structure of the underlying rocks will be pointed out. The 
boundaries between these areas do not follow exactly the out- 
crops of the strata which form the divides between them, for 
short streams which belong to the area underlain by Hudson 
River beds, for example, may rise in the Niagara beds and flow 
eastward across the dip of those beds for a short distance before 
entering the area of the Hudson River strata. The same is true 
of streams belonging to the central basin, which rise at the top 
of the Knobstone escarpment. On the other hand, the streams 
