Tertiary and Quaternary Baseleveling. — Upham. 237 
south to north, the processes of baseleveling were at work 
through the vast duration of Tertiary time, cutting down the 
plains far below their original surface. But here and there 
isolated areas of hills and even mountains remain, consisting 
of remnants of the horizontal Cretaceous strata which else- 
where have suffered erosion. 
The most noteworthy eastern highland area of this kind is 
the Turtle mountain, lying in the north edge of North Da- 
kota and the south edge of Manitoba, its extent on the inter- 
national boundary being about 40 miles, with two-thirds as 
great width. This high tract, diversified by many subordi- 
nate hills and short ridges, 50 to 300 feet above adjoining 
depressions, rises with a massive general form suggesting, as 
seen from some distant points of view, the rounded back of a 
turtle; but as seen from the south or north, its many hills 
and buttes present a serrated outline. Its altitude above the 
surrounding country is 300 to 800 feet, the summits of its 
highest hills being about 2,500 feet above the sea. Beneath a 
veneering of glacial drift, which is in large part morainic and 
generally strown with many boulders, averaging perhaps 50 
to 75 feet in thickness, Turtle mountain consists of nearly 
horizontally bedded Laramie strata, chiefly shales, with very 
thin seams of lignite. At or below the base of this highland, 
the freshwater Laramie formation rests on the marine series. 
which comprises the Fox Hills sandstone and Fort Pierre 
shales, the two great shale formations being separated by a 
sandstone stratum which outcrops in North Dakota on Ox 
creek and Willow river and on the Souris river between Minot 
and its most southern bend. A thickness of not less than 500 
to 1,000 feet of the Laramie and Montana (Fox Hills and 
Fort Pierre) strata has been carried away from the surround- 
ing eastern part of the plains. 
Westward the depth of the Tertiary baseleveling was 
greater. Around the Highwood and Crazy mountains, in 
central Montana, according to Prof. W. M. Davis* and Dr. J. 
E. Wolff, f the erosion of the plains has a vertical extent of 
3,000 to 5,000 feet. Perhaps the most striking evidence of 
*Mining Industries of the United si;iirv. Tenth Census, vol. xv, pp 
{10, 737, 745. 
fBulletin, Geol. Society of America, vol. iif, IS02, pp. 145-452. 
