410 The American Geologist. Tune, 1892 
DELTAS OF THE Hupson AND MOHAWK VALLEYS —The interesting let- 
ter in your last number (p. 344) by Mr. Taylor, and his able article in the 
American Journal of Science for March, give very clear descriptions of 
deltas in the Mohawk valley and of shore lines on the upper Laurentian 
lakes, which he refers to the presence of the sea since the recession of 
the ice-sheet. While thanking him for the excellent descriptions of these 
deltas and shore lines, I wish to offer an alternutive view, showing how 
the Hudson and Mohawk deltas may be well explained by a glacial lake, 
which seems to me to be the only explanation consistent with the com- 
plete absence of Champlain or postglacial marine fossils from these val- 
leys and from all the area of the great Laurentian lakes. This explana- 
tion appeals to the receding continental ice-sheet as the northern and 
northeastern barrier of great glacial lakes in the northeastwardly descend- 
ing St. Lawrence basin, until the farther departure of the ice admitted 
the sea into the then depressed St. Lawrence and Ottawa valleys and 
basin of lake Champlain. These glacial lakes and the Champlain de- 
pression of the country northward have been. demonstrated by the work 
of Gilbert, Chamberlin, Leverett and others. 
On what portion of the St. Lawrence basin did the ice-sheet continue 
latest as a barrier of lake Iroquois, the glacial lake which outflowed 
through the Mohawk valley, and afterward by the way of lake Cham- 
plain to the Hudson? This is answered by finding where the ice at the 
time of its departure was thickest upon the St. Lawrence valley, so that 
its latest movements were thence southwesterly toward the lake Iroquois 
and easterly toward the gulf of St. Lawrence; and this area, as shown by 
-the directions of glacial striz and transportation of the drift, was the 
vicinity of Quebec. 
When the ice blockade was removed, the northward depression of the 
land, which had been slowly rising (as I have shown in Bulletin, G.S. A., 
vol. ii, pp. 258-265), was still sufficient to permit the sea to flow in where 
a great extension of lake Iroquois had previously existed. The incur- 
sion of the sea, at a somewhat lower level than had been held by the 
glacial lake, reached to the south end of lake Champlain, to the Thousand 
islands at the mouth of lake Ontario, and to Pembroke in the Ottawa 
valley, seventy-five miles above the city of Ottawa. To these limits the 
marine fossils of the Champlain submergence are present in beds of clay 
and sand overlying the till. Their maximum observed hight above the sea 
is 520 feet at Montreal, from which their upper limit declines southwest- 
ward to about 250 feet near the mouth of lake Ontario, southward to 
probably a less hight at the south end of lake Champlain, and eastward 
to zero in Nova Scotia. The frequent occurrence of Champlain marine 
fossils to these limits shows the formerly much increased extent of the 
culf of St. Lawrence; but on the other hand the absence of such fossils 
westward from the mouth of lake Ontario and in the Hudson and Mohawk 
valleys marks areas where the shore lines and deltas of former bodies of 
water are referable to glacial lakes, not to the sea. If a strait of the 
ocean had occupied the Hudson valley at the time of the Champlain 
submergence, tidal currents must have swept to and fro through the val- 
