Mil J.B.TYRRELL — POST-TERTIARY DEPOSITS OF MANITOBA. 



ridge thai forma the western side of the Duck mountain, and which was also 

 the eastern shore of the lake, a wide, level, alluvial plain or lake bottom may 

 l>f seen stretching westward to the limits of vision. 



But by far the Largesl and most important of these ancient post-glacial 



lakes is that named Lake Agassiz h\ Mr. Warren Qpham, and which once 

 occupied the Winnipeg basin and the valley of Red river. In its bed was 

 deposited the rich alluvial clay that is now enabling Manitoba to take its 

 place as one of the foremost wheat-producing areas in the world. 



Ancient Beaches. — I shall not now discuss the altitude, length, and depth 

 of these lakes; but a few words may be said of the beaches that at various 

 times formed the shore lines tor the gradually receding waters. 



The existence of the old shores of Lake Agassiz was clearly pointed out by 

 Professor H. Y. Hind in L859, but their relative heights were not at all 

 understood by him. Of late years M r. Warren Upham has carefully studied 

 these beaches from Lake Traverse, at the south end of the Red river valley. 

 in a short distance north of the 50th parallel of north latitude. In the 



w led district further north, and one hundred and fifty miles north-north- 



wesl from where the old lake beaches cross the international boundary at 

 the foot of the Pembina escarpment, several gravel ridges were located by 

 the writer on the northern face of the Riding mountain, close to the hanks 

 of Ochre river, a small stream ilowing into Lake Dauphin. The heights of 

 these ridges are respectively 1,215, 1,115, and 1,025 feet above sea level. 

 From Ochre river they were followed for eighteen miles in a northwesterly 

 direction, at the end of which distance the highest one runs along the summit 

 of a steep escarpment one hundred feet in height, while the one below it i- 

 continuous with the line of the base of the cliff. The face of the cliff is now 

 overgrown with trees, but a gulley that cuts back into it shows it to be com- 

 posed of the white limestones and chalk-marls of the Niobrara subdivision 

 of the ( iretaceous. 



The sequence of events is here very beautifully shown: For a considers 

 ble time the lake atood at the level of the highest of these beaches, and the 

 land -loped gradually beneath the surface of the water. The lake then fell 



more or less rapidly a hundred feet to the Qexl lower shore line, and must 



have stood at this level for a long time, sufficiently long at all events to 



allow the waves t" <ut a cliff of limestone one hundred feet in height from 



what was before a gradually declining Burface. 



From this chalk cliff, which formerly must have -i I out boldly a- a 



conspicuous landmark on the shore of Lake A.gassiz, coasl ridges were again 

 followed and crossed al interval- in travelling uorthward to Valley river. 

 This stream How- in a wide depression separating Duck from Eliding mount- 

 ains, 'lie- highest beach ridge seen on it- hanks has an elevation of 1,280 

 feet above the sea, hut above this is an extensive sandy plain covered with 



