Page:Popular Science Monthly Volume 87.djvu/241

Rh features of the formation. Our camp lay near the only available waterhole, in an otherwise dry canyon, tributary to Buck creek, which forms the eastern boundary of the area. From Buck creek the land rises gradually to the summit of the divide, whence it falls away to the level of Lance creek on the west. On the eastern slope the strata, which dip toward the west at an angle of about ten degrees, form a succession of outcrops one above the other as one ascends the hill, so that they may be read in orderly sequence beginning with the oldest in the point of time. Beyond the divide the dip of the strata and the slope of the ground coincide so that the revealing outcrops are absent. To the east of Buck creek, on the other hand, on either side of the little canyon wherein our

 In the foreground and middle distance the strata are of marine origin—Pierre and Fox Hills formation—the fresh-water Lance sediments lying beyond.

camp was pitched, the rocks, while still late Cretaceous, are older than the Lance formation and of marine origin, for in them the shells of ancient sea-creatures are abundant.

Geologists tell us that during Cretaceous time the continent of North America was covered in part by an inland sea having its outlet to the north into the Arctic Ocean and on the south into what is now the Gulf of Mexico. Along the western shores of this Cretaceous sea were long stretches of low-lying lands gradually rising toward the west to the region of the Rocky Mountains, then in their nascent state. The shore lands, which rarely extended much above the level of the sea, were