Post-Glacial Submergence. 443 
point, as well as to the west, it is clear that the Grand was not 
the agent of excavation. 
I have heretofore* adopted the theory that this central val- 
ley owed its origin to the presence of a strong glacial stream, 
supplied by the wastage of the Saginaw valley glacial lobe, as 
Jaid down on Prof. Chamberlin’s glacial maps. If, however, 
this territory was deeply submerged during the period of 
glacial disappearance, no such glacial stream could have existed. 
Dr. Spencer hintst that the direction of the drainage of this 
region aiter its emergence was towards the east, but that it 
was afterward reversed by the eastward differential elevation 
of the country. This conclusion is based on the differential 
elevation of the Ridgeway beach, which he calculates at a 
little less than one foot per mile across the region in question. 
“From Grand Rapids to Pewamo,” he says, “‘ the beach passes 
through a strait between high lands on both sides,” althongh 
it is not identified at any point between the places mentioned, 
an interval of fifty miles. Let us compare the elevation of 
this beach with other known elevations. The elevation 
of the D., L. & N. railway track at the Ionia station, on 
the valley bottom, is 658 feet. The general level of the 
country immediately adjoining the valley margin is 50 feet or 
more higher, making the elevation probably more than 710 
feet. The nearest point at which the beach has been identified 
is at Pewamo, 12 miles to the east, where its elevation is 
placed at 724 feet. As the beach descends to the west at the 
rate of nearly one foot per mile, its elevation at Ionia must be 
about 10 feet lower, or 714 feet. This practically coincides 
with the level of the valley margin, as above calculated. Now 
the declivity of the central valley throughout the 50 miles is a 
little less than one foot per mile-—almost exactly equivalent to 
the westward slope of the Ridgeway beach. It follows, there- 
fore, that when the beach was formed,.the river valley through- 
out its whole length was also level, and if it then existed it 
was filled just even full of water. 
It may be contended that it was at this period that the 
valley was excavated. The drainage from the adjacent terri- 
tory on either side would be sufficient to create a gentle cur- 
rent, and the direction of flow would necessarily be to the 
east, toward the open sea. However, as the eastern and 
western bodies of water were joined into one at the north, the 
current must have been extremely feeble—quite incapable of 
performing the work. Besides, I have found evidence which 
® “Observations along the Valley of Grand River,” Am. Geol., Nov., 1893 ; 
and “Drainage Systems of the Carboniferous Area of Mich,” Am. Geol., Nov., 
1894. 
+ This Journal, March, 1891, p. 208. 
