132 J. W. SPENCER ON 



been modified by the drawing off of the waters, which process has lowered the lake level 

 116 feet below the lowest point of the Iroquois Beach, at Hamilton. 



(13.) Age of the Iroquois Beach. — "While the valley of Ontario antedates the 

 Pleistocene period, although broadened by the waves of the lake itself, there is no 

 evidence pointing to the age of Lake Ontario being anterior to the epoch of the Drift. Nor 

 do we know that a smaller separate body of water existed when the barrier of the lake 

 was less than at present. Lake "Warren, itself, is older than Lake Iroquois yet it is more 

 recent than the deposit of the stony drift clays of the last great Ice epoch, and of the 

 stratified clays and sands, of the so-called Modified Drift, except such as have been 

 deposited from its waters. 



The Iroquois Beach marks the boundary of Lake Ontario, at the time of its most 

 perfect development, at which time there was probably a shallow overflow to the sea 

 by way of the Mohawk and Hudson valleys. 



The valley of the Mohawk had not been a pre-Pleistocene outlet for. the ancient basin 

 of Lake Ontario, as it is a narrow rock-bound gorge over 900 feet above the bottom of the 

 basin, whilst the bed of the St. Lawrence channel was '700 or 800 feet lower. 



This additional outlet by the Mohawk valley was only a coincidence, as the 

 continent was rising, like many other similar southern overflows of the more ancient 

 Lake "Warren. 



It is difficult to assign an exact age in years. Yet the commencement of the Iroquois 

 epoch does not date back very many millenniums. "We know the rate of terrestrial eleva- 

 tion upon the coast of Scandinavia, where the maximum is five feet per century. The 

 coast of the Bay of Naples is sinking at four feet per century. At other places the rate 

 of movement is known to be less. Since the Iroquois epoch, the beach has been lifted 

 up 700 feet near "Watertown, N.Y., where is to be found the maximum amount of elevation. 

 Assuming the rate to be that of the maximum known change u.pon coast of Sweden 

 viz., five feet per century, Lake Iroquois commenced a separate existence about 14,000 

 years ago. Still, we may have to largely increase this time. Again, Niagara Falls com- 

 menced their history after the beginning of the Iroquois epoch, but then with a fall of 

 only 200 feet, as the Iroquois Lake level was 138 feet higher than that of the modern lake, 

 when the waters of the Erie basin were precipitated directly into it. Hence, the reces- 

 sions of the canon would be slower than now with the modern difference of level between 

 the lakes amounting to 32G feet. The mean modern recession ' is 2 4 feet per annum. 

 This would indicate a lapse of about 15,000 years. But owing to the slower recessions of 

 the shallower canon (if the volume of water has been constant), on account of the 

 diminished fall, and the relatively smaller amount of underlying shales, the age will have 

 probably to be increased to 24,000 years. Thus we get two rough approximations of the 

 lowest age of the- birth of Lake Iroquois at 14,000 or 15,000 years with a probable increase 

 to nearly double this term. 



As the precipitation in the lake region is in excess of evaporation, as the deepening 

 of the outlet of Ontario is very slow, and as the elevating forces in the region of the 



' Computed from the Surveys of 1843, 1875, 1885. See Report of R. S. Woodward's paper, before A. A. A. S., 

 Science, 1886. 



