•514 The American Geologist, November, 1893 
Jurassic types would, under any 1<^> conclusive evidence of 
its Cretaceous age warrant its reference to the Jurassic. 
This section also extends the recorded range of the follow- 
ing fossils: Cerithium bosquense from the Texana beds t<> 
the Fort Worth limestone.* Cgjprimeria crdssa from tin 
Texana beds to the Fort Worth limestone, and Ezoyyra plexa 
from the Kiamitia clay to the top of the Fort Worth lime- 
stone. Furthermore, it gives the horizon of Diplopodia 
streeruvitzii and Plicatitla incongrua, which were hitherto 
uncertain. 
THE PLEISTOCENE ROCK GORGES OF NORTH- 
WESTERN ILLINOIS. 
By Oscak H. Hershey, Froeport, 111. 
Cause of formation of gorges. — The portion of northwestern 
Illinois extending from the glacial boundary on the southeast 
side of the "driftless area," east and south 80 to 50 miles r 
is underlain by one of the oldest drift sheets in North Amer- 
ica. Its formation probably dates from the time of the max- 
imum extension of glacial ice in the Mississippi valley, and 
hence it is a portion of what is commonly known as the 
"earlier drift.' 1 After the ice which formed this drift sheet 
had disappeared from the region, the land remained uncov- 
ered for a period of time during which the till sheet was deeply 
oxidized and leached and a soil formed at its surface- 
Following this was a depression of the land, probably some 
hundreds of feet and the deposition of a bed of silt or loess 
over the till. No very important changes have been effected 
in the region since then, as the ordinary agencies of erosion 
and soil formation have been at work continuously to the 
present time. 
Northwestern Illinois is a comparatively hilly region. From a 
partially completed basal plain of erosion, the streams since ear- 
ly Tertiary time have excavated broad valleys, several hundred 
feet in extreme depth, with gently sloping sides, deeply cut by 
ravines. Glacial abrasion of the rock surface has been slight, 
mn enough to destroy the chief features of the preglacial to- 
pography: and the amount of drift, especially of the ground 
♦Determined by F. W. Cragin. 
