The Development of Mississippi Volleij. — Hershey. 263 
but rarely reaches the indurated rock. This portion of its 
course is post-glacial in age. At St. Anthony falls, the 
stream plunges or did plunge before the arrival of civiliza- 
tion, over a ledge of Trenton limestone, and thence for eight 
miles Hows through a narrow, steep-sided gorge, entirely post- 
glacial in age. At Fort Snelling, it enters the valley of the 
Minnesota river, and immediately makes a sharp bend toward 
the northeast. It flows thence past the city of St. Paul in a 
well defined canon valley which averages 3,000 feet in width 
and 100 feet in depth. Its walls are precipitous blulTs of St. 
Peter sandstone capped by a 25-foot stratum of hard blue 
Trenton limestone. Their sharp cut outlines are due largely 
to glacial and perhaps post-glacial erosion. Just below or 
east of the city, the narrow canon valley suddenly gives wa}' 
to a much broader and older apj^earing canon valley which 
has a trend toward the south-south-east. This latter valley 
averages two miles in width and has sides mantled and often 
deeply buried under drift. It is undoubtedly the old or pre- 
glacial valley of the Mississippi river. Almost as certainly 
is the narrow, new appearing canon valley which passes St. 
Paul not the pre-glacial trough through which the ancient 
Mississippi held its way. It is in the direct line of continu- 
ation of the Minnesota river valley and may formerly have 
been occupied by that stream. It« great depth below the 
present river level (about 200 feet) seems to place it in the 
category of pre-glacial valleys, but its comparative narrow- 
ness, especially the short distance (about two and one-fourth 
miles) between the known areas where the soft Tre?\ton 
shales are in considerable thickness on either side of it, some- 
what negative a reference to it as the pre-glacial valle}^ of 
the Minnesota river. It is possible that it may yet be shown 
to be inter- glacial in age. 
Disregarding the much narrower valle}' which enters it 
from the west, the pre-glacial caiion valley of the Mississippi 
river, ends abruptly at the southeastern corner of St. Paul. 
The high upland area which trends north and south on its 
eastern side at some distance from its immediate border, 
continues without a change for many miles to the north 
passing to the east of lake Phalen. Although deeply covered 
with drift, it is undoubtedly based on an upland area of rock. 
