PLATEAU OP PEMBINA MOUNTAIN. 93 



In the escarpment ami plateau of Pembina Moimta'm. — Sections cut by 

 the head streams of the Goose and Turtle rivers, in the highhmd between 

 the Red River Valley and Devils Lake, and the similar erosion of the 

 Pembina Mountain by the branches of Park River and by the Tongue, 

 Little Pembina, and Pembina rivers, show that, beneath the drift, this long- 

 escarpment and the plateau which it bounds consist of dark, sandy shale, 

 horizontal or nearly so in stratification, and nearly uniform in character for 

 a great thickness. Gravel of this shale abounds in the channels of the 

 streams for many miles east of the escarpment, and is found sparingly in 

 the drift southward to the Coteau des Prairies. It is commonly called 

 "slate" by the people of this region; but no portions observed possess 

 the hardness and texture deserving this name, and no slaty cleavage is 

 exhibited. 



The highest outcrops of this formation seen by me, close to the west 

 shores of Lake Agassiz, are on the plateau that extends westward from the 

 top of the Pembina Mountain, where this is channeled by the North Branch 

 of Park River in the vicinity of Milton. Along a distance of 5 miles from 

 northwest to southeast through the north part of township 150 north, range 

 57 west, this stream cuts 75 to 125 feet into the shale, its top being 1,500 

 to 1,550 feet above the sea. It is overlain by only 5 to 25 feet of till, 

 which continues ecpially thin, as shown by watercourses and wells pene- 

 trating to the shale, for 15 to 30 miles westward and northwestward, and, 

 excepting two or three morainic belts, upon all the country southwest to 

 Devils Lake. The surface about Milton is moderately undulating and 

 rolling, with the crests of its higher portions 25 to 40 feet above the 

 depressions. From this plateau an irregular descent, amounting to about 

 100 feet per mile, occupies the east part of this township and the edge of 

 that next east, falling in 3 miles to the upper shore-line of Lake Agassiz, 

 which here is approximately 1,200 feet above the sea. 



The deepest i)art of the gorge of the North Branch of Park River is in 

 sections 4, 9, and 10, township 159, range 57, where it is 125 to 150 feet 

 deep and from a quarter to a half of a mile wide, with numerous fine 

 exposures of the shale from the base to the top of the bluffs, except the thin 

 capping of drift. In these i)laces the stream, flowing at the base of the 



