This outer slope varies greatly in width and is underlaid by older sedimentary rocks, cut in almost every direction by narrow deep canons.  This feature covers nearly the whole of the western half of the Hills proper, where erosion has been less active on account of its distance from the main channels of drainage.  Usually, from the broken interior edge of this slope or sedimentary plateau one descends a bluff or escarpment, and enters the central area of slates, granite, and quartzites, which is carved into high ridges and sharp peaks cut by many narrow and deep valleys and ravines and generally thickly timbered with the common pine of the Rocky Mountains.  Toward the south, about Harney Peak, the surface is peculiarly rugged and difficult to traverse.  Toward the north, also, about Terry and Custer peaks, a smaller rugged surface appears; but in the central area between and extending west of the Harney range is a region which is characterized by open and level parks much lower than the surrounding peaks and ridges.”

The Archaean rocks which form the core of the Hills mark the center of the various uplifts which have attended their formation and controlled their history.  The coarse granite of Harney Peak indicating that, as the central point of the earliest upheaval, and the three porphyries known as rhyolite, trachyte, and phonolite, showing the uplifts of later periods to have had their centers a little more to the north, but the entire area is said to be only about sixty miles long and twenty-five miles in width.  It is exceptionally rough and mountainous, and consequently has great charms for the lover of fine scenery.  Erosion has only partially denuded the peaks of the sedimentary rocks through which they were thrust up, or by which they were overlaid during the earlier part of several subsequent periods of submersion.  The Hills, in these remote times, led but a doubtful and precarious existence, being now an isolated island rising out of a shallow sea, and then, owing to a general subsidence, submerged in the ocean to so great a depth that even Harney Peak is supposed to have almost, if not entirely, disappeared.  This up and down motion continued at intervals until the Fox Hills epoch of the Cretaceous Age, at the close of which the sea retired forever from that portion of the country.  In the next epoch fresh water work began and extensive marshes were formed, with an abundant growth of vegetation and reptiles.  There was also much volcanic violence which resulted in the fine scenery in the north end of the Black Hills, and probably opened the fissures to form Wind Cave, the Onyx Caves in the southern hills and Crystal Cave near the eastern edge toward the north.  This was near the close of the Cretaceous Age.  But here is a point on which the best authorities who have studied the porphyry peaks, have failed to agree; Prof.  N.H.  Winchell believing that the intrusion occurred, probably, during the Jura Trias, but as Cretaceous beds, of more recent date, are found to have been distorted by the outflow, it seems that Professors Todd, Newton and Carpenter hold the stronger position and that the later time is correct.