Page:Handbook of Meteorology.djvu/143

 Wind-blown Dust.—In regions of sparse vegetation, where the ground is bare, enormous amounts of loose rock waste are moved hither and thither by the wind. The increase of the carrying power of the wind with increment of velocity is almost beyond belief. When the velocity of the wind is doubled its carrying power is increased sixty-four fold. In regions of loose rock waste the wind becomes a wonderful physiographic factor. The broad, intermontane valleys of the plateau region have been filled with rock waste, much of which is wind-blown; and the floors of the deeply filled valleys have been made level by wind-blown dust. The plains to the eastward of the Rocky Mountains are deep with wind-blown dust. More dust and rock waste is deposited in the rivers of this region than they are able to carry. Platte River, popularly described as “a mile wide, an inch deep, and bottom on top,” is an instance of a river drowned by the rock waste which it cannot carry.

Winds blowing steadily for centuries have carried fine rock waste from the Gobi far into eastern China, choking the gorge of the Hoang in places with wind-blown dust more than ioo feet deep. The loess deposits in the lower course of the Hoang are also of wind-blown dust, which has been dumped into the river in quantities greater than the river could carry. In 1851 the channel had become clogged to the extent that the river broke its banks near the city of Kaifeng, abandoned the old channel to the delta of the Yangste, and made a new channel to the Pechili. The sediment with which the river had clogged its channel was wind-blown dust. In general, the action of the wind in unswarded regions is one of leveling. It wears away the high spots and fills the low spots.

In regions of generous rainfall, the surface is covered with vegetation to the extent that very little rock waste is exposed to the action of the wind. About the only physiographic action consists of the formation of sand dunes to the leeward of ocean and lake shores. In various instances these have gradually traveled a distance of several miles inland, ceasing to advance when growing vegetation has anchored the sand in place.

In cities and much-traveled rural districts, the wind-blown dust is picked up mainly from dirt streets, school playgrounds, and other unswarded areas. The wind-blown dust from these consists chiefly of loose dirt, paving material, garbage, finely