Page:Popular Science Monthly Volume 80.djvu/213



HOUSANDS of years ago when the forces of nature were at work shifting and gradually shaping the features of the Great Plains, large areas of Tertiary sandstones were exposed in Dakota, Wyoming, Nebraska, Kansas and other parts of the western plains. As topographic features were slowly evolved, these sandstones, being young and soft, readily yielded to the eroding action of the elements and were reduced to light, fine-grained sand. In addition to this sand formed in situ probably considerable amounts of other sands were washed or blown into the region from farther west. Great quantities of the sand thus formed were caught up by the wind and heaped into mounds that finally grew to be large sand dunes extending in long ranges and ridges for many miles over the sandstone beds. Thus were the Sand Hills of the Great American Desert formed in the days preceding the advent of plants or men into the regions now characterized by the billowy hills covered with the bunch-grasses and their associates.

The Sand Hill landscape in these early days was probably a restless maze of wandering sand dunes. In later years certain plants crept in from the surrounding plains, only to be uprooted and blown away. After many such invasions some individuals finally succeeded in maintaining a foothold in the more protected portions of the hills. Notwithstanding the terrible conditions imposed by an arid climate and a continually shifting soil, vegetation continued to spread to other areas from these primary centers of establishment.

Some time after the Sand Hill flora had gained a lasting hold upon the dunes and the greenish hue of vegetation had spread over the great expanse of hills, enormous herds of bison came charging into the region