86 THE GLACIAL LAKE AGASSIZ. 



are known in this interior ])ortion of the continent. Since the early stages 

 of the Laramie epoch, the lied of the Cretaceous ocean now forming- the 

 great beh of plains that stretches west from the lake district of Manitoba, 

 the Red River Valley, and eastern Nebraska and Kansas, to tlie Rocky 

 Mountains, has not been submerged beneath the sea. 



FORT PIERKE SHALES WEST OF LAKE AGASSTZ. 



The ascent upon Cretaceous strata, at the south a massive ridge and at 

 the north a liold escarpment, on the western border of tlie Aalley in which 

 Lake Agassiz la\', called in successive portions the Coteau des Prairies, 

 Pembina, Riding, and Duck mountains, and the Porcupine and Pasquia 

 hills, has mostly so thick and continuous a covering of glacial drift that 

 onlv few exposures of the underlying strata are seen, chiefly where chan- 

 nels have been eroded by streams. Tlii-oughout their extent of 800 miles 

 the ridge and escarpment appear to consist mainly of the Fort Pierre for- 

 mation, pi-esenting a thickness of several hundred feet of dark shales, mostly 

 soft and somewhat sandy. Under the Fort Pierre beds are similar shales 

 belonging to the Niobrara and Fort Benton formations, succeeded below by 

 the Dakota sand.stone. The overlying drift varies coinmonlv from 10 or 20 

 feet to more than 100 feet in depth. 



Southivestern Minnesota and, the Coteaii des Prairies. — (Jutcrops of the 

 Cretaceous strata on the Coteau des Prairies, and in Minnesota east of this 

 highland, are rare and usually of small extent, both in area and in the 

 vertical thickness exposed; but these beds are also occasionally penetrated 

 by wells near the east base of tlie coteau, and at the mission school, 1^ 

 miles north-northwest of the Sisseton Agencv, on the eastern slope of the 

 coteau and about 1,500 feet above the sea, a well boring passed through 

 the drift and entered soft shale or clay, probably the lower part of the Fort 

 Pierre formation, at the depth of 138 feet. The outcrops mentioned east 

 of the Coteau des Prairies appear to belong mostly to the lower divisions 

 of the Upper Missouri series. They include sandstones of the Dakota 

 formation on the Cottonwood River, from its mouth to a distance of 30 

 miles west, yielding numerous species of fos.sil leaves; beds of shale, with 

 thin seams of lignite and lignitic clav, occurring hi the bluffs of the Minne- 



