472 THE GLACIAL LAKE AGASSIZ. 



sandy deposits at the base of Stony Mountain on its north and south sides 

 are attributable to erosion by the lake, there only a few feet dee]), at the 

 time of formation of the Niverville beach. Its level was 15 to 20 feet 

 above the surface where the city of Winnipeg is built. 



Numerous observations of the Niverville beach have been made by 

 Mr. Tyrrell on the shores and islands of Lake Winnipeg. Its occurrence 

 on Black Island, about 15(1 miles north of the international boundary, is 

 described by him as follows, excepting that his later determination of the 

 height of the beacli as GO feet above Lake Winnipeg is substituted instead 

 of his previous estimate, which was 20 feet lower: 



At Ox Head, uear tlie northeastern extremity of Black Island, an ancient beach 

 is very conspicuous at about 60 feet above the water. On the south side of the island 

 the beach is marked by a line of sand dunes, and on the north side a sandy terrace 

 rises gently to a height of 60 feet and ends abruptly at the foot of a steep slope thickly 

 strewn with bowlders. On ascending this slope the land is found to rise to a height 

 of 100 feet above the lake and its summit to consist of an irregular aggregation of 

 knolls, thickly strewn with large bowlders of gneiss, very few or none being derived 

 from the immediately adjoining or underlying Keewatin schists. This ridge is the 

 summit of the Black Island moraine, which would seem to have been dropped here 

 when the higher parts of the island were above the surface of Lake Agassiz, as there 

 is no sign of water action on the moraine above the line of the 60-foot beach. It is 

 possible that the moraine may have been deposited about the water level, and that the 

 water afterwards rapidly receded to a height GO feet above the present lake. ' 



Instead of the view taken by Mr. Tyrrell, however, concerning the 

 depth of Lake Agassiz here when the Black Island moraine was formed, I 

 believe that it was deposited in water 600 to 700 feet deep, bordering the 

 ice front, contemporaneously with the foi-mation of the Herman or Norcross 

 shore-lines. The morainic accumulations, lying thenceforward at the bot- 

 tom of Lake Agassiz, could not have been exposed to erosion by its waves 

 until the very late change of its northward outlets, by which the lake fell 

 about 50 feet between the Stonewall and Niverville beaches; and this 

 reduction appears to have taken place so quickly that no beach ridge nor 

 eroded escarpment was made on the upper part of Black Island. 



At the Grand Rapids of the Saskatchewan, according to Mr. Tyrrell, 

 the tramway of the portage, which is some 4 miles from the mouth of the 



' Am. Geologist, Vol. VIII, p. 25, July, 1891. 



