NORTHEASTERN LIMIT OF LIMESTONE DRIFT. 137 



were observed of all sizes up to 10 or 12 feet cube. Generally as large a 

 proportion as 99 per cent of the bowlders exceeding 1 foot in diameter 

 consists of Ai-clieau granite, gneiss, and schists, being derived from the 

 Archean area on the northeast and north. With these are occasional lime- 

 stone blocks, derived from the belt of Paleozoic limestones, constituting on 

 the average perhaps nearly 1 per cent of the large rock fragments of the 

 di'ift. The bedded and jointed character of the limestones has prevented 

 their supplying many large bowlders in comparison with the more massive 

 crystalline Archean rocks, while yet usually about half of the smaller 

 cobbles and pebbles in the till and in gravel and sand deposits are from 

 these Paleozoic limestones. Upon the Cretaceous area a considerable pro- 

 portion of the gravel and cobbles is derived from the Fort Pierre shale, 

 but this formation supplies no large blocks. 



Northeastern limit of limestone drift. — East of Lake Winnipeg and 

 northeast of a line drawn from this lake southeastward by Lac du Bonnet 

 on the Winnipeg River and across the Lake of the Woods to the west end 

 of Rainy Lake and onward to Vermilion Lake, both bowlders and gravel 

 of limestone are absent or exceedingly rare. This line probably marks 

 the farthest extent ever attained by the glacial currents which moved 

 south-southeast in the vicinity of Winnipeg and at Black Bear Island, near 

 the Narrows of Lake Winnipeg, carrying ddbris from the limestone region 

 of the Manitoba lakes. 



It is also very remarkable that the same line divides an area of very 

 thLi di'ift on its northeast side from the area of very thick drift which 

 thence extends southwestward across all western Minnesota, the southern 

 part of Lake Agassiz, and the region of the Sheyenne and James rivers 

 to the Missouri Coteau. 



Localities of very abundant and large bowlders. — The following localities 

 may be mentioned as having especially abundant bowlders : On the slope 

 of the Pembina Mountain, in township 3, range 6, Manitoba, between 

 Morden and Thornhill, very plentiful and large bowlders are spread upon 

 an area of several square miles, as noted in the description of the Tintah 

 beaches. The sides of Star Mound, Manitoba, especially those facing the 

 north and northeast, are sti'ewn vrith a multitude of bowlders, nearly all 



