Page:Popular Science Monthly Volume 80.djvu/215

Rh mile distant. Sometimes instead of the valleys being separated by a range of round-topped hills this is accomplished by a continuous rounded ridge. The sides of these hills are often very steep, making difficult the direct passage over from one valley to another.

In the regions of widest valleys the ranges of hills often show a succession of higher hills as one passes back from the valley to the highest points on the divide, which may be from 300 to 400 feet above the level of the valley. In the regions characterized by short valleys and basins the general landscape is strikingly different because in such places the hills rise on all sides without any regularity. Low hills, intermediate hills and high hills are all closely associated, with no long separating valleys. The result is a very abruptly rolling surface with rounded or oblong depressions of varying depth, with the rounded or conical dunes above. There are places where this sort of topography stretches in all directions as far as one can see.

As the name implies, the hills are composed of sand. This sand is of a light straw color (not white) composed mostly of fine grained quartz. The purest sand is found in the newest soil areas such as in "blow-outs" or other places where the overlying vegetation has been completely removed by the wind. In many places, notably on the river flats and in the numerous thickets scattered throughout the hills, there is a copious admixture of organic remains and so the surface soil in such places is a rich black sandy loam and is very fertile. But the characteristic soil of the region as a whole is the pure dune sand composed of very fine particles. As to the chemical nature of the sand, the following table shows it to be very high in insoluble mineral matter and very low in soluble organic or inorganic plant-food materials.

The following table shows the size of the soil particles in per cent., and the average of three determinations from different stations in the Sand Hills: