96 THE GLACIAL LAKE AGASSIZ. 



About three-quarters of a mile east from the fish trap, this snale is 

 exposed to a height of 75 feet above the Pembina River in a bluff on its 

 northeast side. Its highest outcrop examined by me in this vicinity is 

 Heart Mound/ a peculiar hillock with very steep sides and smoothly 

 rounded top, situated on a broad, uneven terrace of Pembina Mountain, 

 near the center of section 6, township 163, range 57, 3 J miles north of the 

 fish trap and 2 miles south of the international boundary. The base and 

 top of this hillock are, respectively, about 1,360 and 1,390 feet above the 

 sea. Some have erroneously supposed it an artificial mound. Glacial 

 drift, containing granitic bowlders up to 4 or 5 feet in diameter, thinly 

 covers its northeast side; but the other sides and the crest of this knob 

 consist of shale similar to that on the North Branch of Park River, show- 

 ing that it is an outher of the Fort Pierre beds that form higher land, drift- 

 covered, about a mile westward. Heart Mound has been left thus isolated 

 by the erosion of these beds from the suiTOundiug area. 



The thickness of the Fort Pierre formation in the northeast part of 

 North Dakota, according to these observations, is at least 300 or 400 feet; 

 but it doulrtless considerably exceeds this, for there is no indication that 

 these exposures mark its upper and lower limits. Wells in Langdon, 17 

 miles northwest of Milton and 1,610 feet above the sea, after passing 

 through only 12 to 15 feet of till, enter this shale. With the underlying 

 Niobrara, and Fort Benton formations, which in this part of the State difter 

 little in lithologic characters from the Fort Pierre, a thickness of 1,403 feet 

 of dark-gray shales was penetrated by the Devils Lake artesian well 

 before reaching the top of the Dakota sandstone, which there is 39 feet 

 above the sea-level. In Fort Pierre outcrops on the North Branch of 

 Turtle River, 1 J miles north of Niagara and about 1,375 feet above the sea, 

 BacuUtes ovatus Say was found in abundance. The eroded eastern edge of 

 the Fort Pierre shale forms the long, high escarpment of the "Second" 

 Pembina Mountain, as the eroded border of the Pembina delta at Walhalla 

 forms the almost equally notable, though much shorter, "First Mountain." 



'Coiumonly called by English-speaking immigrants "the Indian Mound," but more properly 

 named as above, in accordance with the usage of the French voyagers and residents, who, probably 

 translating the aboriginal name, call this mound and the small area of prairie around it La Bale du 

 CcEur. 



