Page:Popular Science Monthly Volume 90.djvu/586

 Their Story

���Sentinel Rocks and

��They relate chapters of tremendous earth movements in the past

��Mushroom pillars of sandstone capped with an iron-stone which has protected them from more extensive erosion

��THE western portion of the United States is full of strange rock forms — lofty natural towers, pinnacles and monuments, many of commanding dimen- sions. Some of the rock towers are composed of relatively soft material, such as sandstone, and their history has been one of rapid erosion and disintegration; others of granite stand seemingly unchangeable, yet their very forms show that they too have been the playthings of the elements.

The light-colored sandstone mushroom- like pillars which are numerous in the vicinity of Monument Park, Colorado, are the visible remnants of a great bed of such sandstone, which in a past geologic age covered this portion of the United States. At intervals through this bed of sandstone there were thin strata or layers of a harder iron-stone, fragments of which are now seen as cappings of the sandstone pillars, pro- tecting them to a considerable degree from erosion. The altitude of these sandstone rocks is about five thousand feet above sea- level, yet they were the ocean's bottom.

At another point, near the Garden of the Gods, Colorado Springs, the rock towers are of a somewhat harder sandstone. The Major Domo, as it is called, is one of the most striking of the sentinel rocks of this strange garden. It is of red sandstone, and one of the remnants of a great bed of stratified sedimentary rock which, in the

��upheaval of the Rocky Mountains, was actually turned on edge. The "grain" of the rock, therefore, now runs vertically whereas originally it lay flat.

The leaning pinnacle surmounting one of the ridges of the West Elk Mountains, in Colorado, is a rock of great strength and resistance to erosion.

. The ridge consists of rock which was loog ago forced up from the bowels of the earth, hot and liquid, and intruded into the layers of sandstone and other sedimentary rocks nearer the surface in a mass several thou- sands of feet thick. It then gradually cooled, consolidated and partially crystal- lized into hard rock. Later, erosion strip- ped off the sedimentary rocks and carved the hard rock into one of the West Elk Mountains, on which may be seen untold numbers of such pinnacles.

The big Bad Lands of South Dakota constitute a large area of very soft clayey sandstone which was once the bed of a great inland sea. It is now a well elevated region and the elements have cut and carved the rock into every conceivable shape. There are human and animal faces and forms, and towers and ruined castles and palaces, a perfect riot of possible and impossible shapes. The disintegration is in comparatively rapid progress and the soft rock is continually scaling and falling away from the columns and pinnacles.

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