Frozen Streams of I turn Drift Border. — Wilson. 365 
channel at Dubuque had a depth at least 132 feet below the 
present low water mark. A number of wells in T. 87 X., R. 3 
W., in Delaware county, indicate a depth of channels reaching 
at least 40 feet, perhaps much more, below the present level 
of the water in the streams. As this would carry them 
through the more durable limestone of the Niagara into the 
soft, easily eroded shales of the Lower Silurian, it is highly 
probable that their general level was no higher above that of 
the Mississippi then than now. That is, their channels were 
100 feet, more or less, deeper than at present. The relation 
of the limestone to the shales is the same as that in the Niag- 
ara river gorge, although the Xew York shales are not the 
equivalent of the Iowa shales in age. 
These half-buried channels are generally quite narrow and 
crowd each other so that there are often several on one square 
mile. The aspect of this region before the ice invasion must 
have been decidedly rugged and picturesque. The underlying 
shales may be held accountable for the depth and narrowness 
of the channels, and every considerable stream must have 
been a Xiagara river in miniature, with nearly vertical rocky 
walls rising from 100 to 200 feet above the water, and with 
falls near the head of the stream, whose hight would depend 
on the power of the stream. 
The meanderings of the bluffs of these streams follow 
courses in two prevailing directions, east by south, and south 
by west, and these are noticeably the directions of the systems 
of joints in the Xiagara limestone. While it is not to be sup- 
posed that these joints had anything to do with determining 
the original location and direction of the streams, yet it 
seems reasonable to suppose that they would fix the angles of 
the streams, would eliminate projecting points not harmonious 
with themselves, and would introduce angles in a course thai 
might at first have been straight, but oblique to their direc- 
tions. This tendency would be greatly strengthened by the 
presence of the treacherous shales beneath. It will readily 
appear that a surface with such bold relief, with narrow, deep 
and angular channels, would offer a high degree of resistance 
to the movement of the glacial ice. 
The South fork of the Maquoketa river lies throughout its 
entire length from ten to twenty miles inside the border of 
