Toronto Interglacial Period.—Coleman. 75 
allowed for. The gentle slopes and rounded contours of these 
valleys prove that the work must have been lengthy. Even 
if the rivers were powerful glacial streams cutting down their 
channels rapidly, as Mr. Upham appears to assume,” this would 
not account for the wideness of the valleys and the gentleness 
of their slopes, which must have been due largely to rain and 
rill erosion. As the valleys are more mature than those now 
being cut through similar material below the Iroquois level, 
we must conclude that their production probably required a 
longer time than has elapsed since the Iroquois lake was 
drained. How long ago this took place is of course very un- , 
certain. If the present shore of lake Ontario is compared 
with the old Iroquois shore they seem of about equal maturity 
and probably required about equal times for their produc- 
tion, which suggests one-half of the time since Niagara began 
to cut its gorge for the erosion of the present river valleys be- 
low Iroquois level. As the age of Niagara is variously esti- 
mated at from 5,000 to 35,000 years, one-half of the time since 
it commenced its work may be any where between 2,500 and 
17,500 years. 
It is then altogether likely that the cutting of the intergla- 
cial river valleys occupied more than 2,500 years, perhaps 
very much more. , 
The length of time required to deposit the 203 feet of 
boulder clay and interstratified material overlying the inter- 
glacial deposits it is not necessary for us to reckon. The time 
limits for the different interglacial events as described above 
cannot, of course, be very sharply defined, but even with low 
estimates the total time demanded amounts to several thous- 
and years; more than Mr. Upham allows for the whole retreat 
of the ice from the northern states. If instead of the smallest 
admissible estimate in each case more liberal but yet thorough- 
ly probable ones are assumed, and a reasonable time allowed 
for a rich and varied forest growth to advance and occupy the 
desert plain left after the first retreat of the ice, the length of 
interglacial time must have stretched to 10,000 or more years, 
possibly to 50,000. 
Let us now turn to a consideration of the relationship of 
the ice sheet to the interglacial deposits. Mr. Upham evidently 
*Am. GEOL., Nov., 1901, p. 315. 
