84 27/ e American Geoioyis'. August, is96 
The same cliH'erencp is observed in the I'eeatonica valley. 
Where it is excavated into the Galena limestone the valley is 
from one to two miles wide; but u])Oii i'ollowiiig it into Wis- 
consin onto the St. Peter sandstone terrane it l)ecomes a nar- 
row canon-like trench. 
It is chiefly to the depth of the gorges that they owe their 
width. In Stephenson county where the l)lutl's rarely exceed SO 
or 100 feet in hight, the valleys are comparatively broad and 
do not resemble gorges. The same system of valleys excavated 
into the same formation in Jo Daviess county, but to a depth 
twice as great, l)ecome narrow-canon-like troughs with many 
rocky precipices. 
The bottojn of these rock troughs is far below the present 
water level. Well-sections in the preglacial valley of the 
Mississippi river at three places have reached the rock-floor of 
the valley at depths below the present low water level as fol- 
lows:* Dubuque, Iowa, 13" feet; Sabula, Iowa, 1-13 feet; 
and Fulton, 111., 101 feet. I have been unable to secure very 
satisfactory well-sections over the deeper portions of the val- 
leys within this district, but several near the south side of the 
Pecatonica valley at Freeport penetrate to depths of 100 feet 
before reaching rock. Outside of the district but within a 
moderate distance from it. there are well-sections which indi- 
cate the rock-bottom of the ancient Kock-Illinois valley as 
follows :f On the border of lake Koshkonong, Wis., 330 feet 
below the Rock river; at Janesville, Wis., 250 feet below the 
same river; and in an abandoned channel at Princeton, 111., 
the rock surface is about 165 feet below the present water 
level. 
It is evident, from the above sections of wells on both sides 
of our district, that the rock-floor of the valleys is at least 
100 feet, and in the extreme northeastern portion possibly 
200 feet, below the alluvial plains of the present streams. I 
have found what I believe to be evidence that the erosion of 
these canon valleys was eifected in two stages. The evidence 
consists chiefly of a rock shelf buried 20 feet under the river 
level at Freeport. A comparison of the topographic forms of 
*From a table published bv F. Leverett, Journal of Geolofjy, vol. in, 
No. 7. 
tChamberliu and Salisbury, Sixth Annual Report, U. S. Geol. Survey. 
