Glacial Lakes. — Upham. 225 
more than a tenth part of the Postglacial period, or, in round 
numbers, probably about 1,000 years.* 
If we thence attempt to estimate the stages of these several 
glacial lakes, starting from professor X. H. YVinehell's com- 
putation of about 8,000 years as the lapse of time since the be- 
ginning of erosion of the gorge between Fort Snelling and the 
Falls of St. Anthony in Minneapolis, we may suppose the ex- 
istence of lake Agassiz, in all its thirty successive levels of de- 
crease in both depth and area, to have ended about 7,000 years 
ago. Lake Warren may be assigned to approximately the same 
duration of about 1,000 years after the St. Anthony gorge be- 
gan to be eroded. With the end of lake Warren, the cutting 
of the Niagara gorge between Lewiston and the present posi- 
tion of Xiagara falls began, and has occupied a less time, there- 
fore, than the gorge below Minneapolis. Lake Iroquois, be- 
ginning at the same date as the X'iagara gorge, lasted perhaps 
1,000 years, ending some 6,000 years ago, which likewise 
marked the time of transition of lake Hudson-Champlain into 
the incipient lake St. Lawrence. In its turn, this latest great 
ice-barred body of water endured probably another 1,000 years, 
to about 5,000 years ago. It ended by the melting of a gap in 
the ice-sheet barrier, admitting the sea to the former lake area. 
Finally, the last remnants of the ice-sheet on the Adirondacks, 
the White mountains and northern Maine, and in the central 
parts of British America, both west and east of Hudson bay, 
may have lingered 1,000 to 2,000 years more, until only 4,000 
to 3,000 years ago. Although these crude and offhand esti- 
mates have no exact value, they seem to give a safe conclusion 
that the closing Champlain epoch of the Ice age was geologic- 
ally short and near our own times, since it extended, indeed., 
into the period of Egyptian or even Greek history. 
Five marginal moraines in central and northern Minnesota 
were formed contemporaneously with lake Agassiz ; and doubt 
less also an equal or larger number of these belts of knolly and 
hilly drift, marking stages in the wavering recession of the ice- 
sheet while if was the barrier of that lake, will be found in 
Manitoba and farther north. Earliest of these moraines in 
Minnesota are two that are merged in the prominent Leaf hills, 
* The Glacial Lake Agassiz, Mon. xxv, U. S. Geol. Survey, 1895, pp. 238- 
244. 
