846 The American Geologist. December, i,h95 
IIkmt cxcMvation. Now, the period of erosion of the ancient 
valleys in northwestern Illinois is limited, being subsequent 
to the period of ('retaeeous baseleveling and previous to the 
glaciation of the region. In short, it corresponds to the Ter- 
tiary era, and I have referred to the valleys then eroded as 
Tertiary valleys. Similarly in the Ozark region the erosion 
of the upper troughs of the streams lirst began wiien the Jura- 
Cretaceous peneplain was uplifted, and continued to the close 
of the Tertiary era. These troughs are the Tertiary valleys of 
southern Missouri, while the nearly imperceptible basins on 
the peneplain are the Cretaceous valleys of the same region. 
QrATKKXAKY VaLLKYS. 
The Pleistocene gorge of the Rock river below the mouth of 
the Kishwaukee in Illinois nrst came into existence on the re- 
treat of the first ice-sheet that had overrun what is now Il- 
linois, and it has been in process of formation, at varying 
rates, ever since. But it was practically completed previous 
to the deposition of the loess of the Mississippi valley. It 
averages, in the portions which are strictl}^ post-Tertiary in 
age, about a quarter of a mile in width and 75 to 150 feet in 
depth. The lower gorge or present valley of the White river 
is a fourth to a half of a mile in width, and averages in Mis- 
souri 100 feet in depth. The cross-section of one is thus seen 
to be approximately equal to the other. Similar Pleistocene 
rock gorges of the Pecatonica river are an eighth of a mile 
wide and 60 feet deep. The lower trough of the James river 
near Galena averages a sixth of a mile in width and 60 feet 
in depth. Pleistocene gorges in the Yellow creek valley are 
about 400 feet wide and 60 feet deep. The lower trough in 
the Flat creek valley averages 600 feet wide and 60 feet deej). 
It is thus seen that the cross-section of the lower troughs or 
immediate valleys of the Ozark streams is greater than of the 
Pleistocene valleys in northwestern Illinois, but the ditference 
is not very great in amount. 
It must be remembered', however, that the gorges of Illinois 
extend through only a comparatively small portion of a 
stream's course, while those of 3Iissouri extend throughout 
the entire length of the stream. But since the Pleistocene 
valleys, although quite narrow as compared with the Tertiary 
valleys of the sam3 region, are from three to twenty times as 
