Pleistocene Hock Gorges, — Hershey. 31 r> 
moraine or till sheet, is not sufficient to obscure the original 
contours of the land. Therefore the exact size and shape of 
the preglacial valleys can be very easily determined. 
The ground moraine is accompanied by a complex system 
of gravel and sand beds of Glacial age, occurring in the form 
of ridges and knolls, and collected into belts which cross the 
country more or less parallel to each other. Their chief de- 
velopment is in the valleys, which they sometimes follow, but 
just as frequently they trend obliquely across, and in quite a 
number of cases they cross a small valley at right angles to 
its course. In this way many of the preglacial valleys have 
Income partially filled, at certain points, with glacial gravels 
and sands. Obviously, on the melting of the ice-sheet from 
northwestern Illinois, and when the gravel beds had settled 
to their permanent positions, small lakes and ponds must have 
come into existence in those valleys which were partially filled 
with drift at some given point. The ponded water, in seek- 
ing an outlet, flowed over the lowest place in the rim of 
its basin, and so determined the future course of the stream. 
Now. it so happened that in quite a number of cases, the 
gravel and sand bed closing the valley rose to such a bight 
that the stream was completely blocked in that direction, and 
began to cut its outlet through the solid rock at the side of 
the-original valley. In course of time an entirely new valley 
was formed, which can be very readily distinguished from 
those which are preglacial in age. 
Rock gorges, by which term are meant very narrow valleys 
having steep, almost precipitous side-, composed of solid rock 
strata, occur in nearly all parts of the district in northwest- 
ern Illinois under discussion, and in nearly every case can 
he traced to the above cause. If each gorge indicates the 
presence, at one time, of a small lake or pond, as it evidently 
doe-, a map of the region made in early Glacial time- would 
probably show a surface thickly studded with small lakes. 
For tin- county of Stephenson it has been calculated that the 
amount of water surface must have been at least one twenty- 
fifth part of its area. 
Description. — The following table has been prepared from 
measurements and estimates of the dimensions of a few of 
these rock gorges : 
