176.Wind Work. -The wind must be considered among the forcesaffecting the earth in its relation to man. Whenever the wind blows over dry land， particles of dust and sand are blown away and deposited elsewhere. The interiors of our houses often become covered with dust blown from the dry streets. Even on ships at sea， thousands of miles from land， dust has been collected.
In volcanic eruptions great quantities of dust are thrown into the air and spread broadcast over the earth. On the highest and most remote snow fields particles of this dust have been found. In the great eruption of Krakatoa， dust particles made the complete circuit of the earth， remaining in the air and causing a continuance of red sunsets for months.
Sand is not carried as far as dust， but at times of strong wind it is often borne for long distances. Even houses， trees and stones of considerable size may be lifted and moved by a fierce wind storm.
The wind-swept detritus has been known even to obstruct and modify the course of streams. Where the wind blows dust constantly in one direction， deposits of great thickness are sometimes made.
In Kansas and Nebraska there are beds of volcanic dust， reaching in some places to a thickness of more than a score of feet and yet there are no known volcanoes either past or present within hundreds of miles. In China there is a deposit of fine dustlike material， in some places a thousand feet thick， which is thought by some to be wind blown. This forms a very fertile and fine-textured soil and supports a great population. Many of the inhabitants of the region live in caves dug in the steep banks of the streams， so firm and fine textured is the material. Wind deposits of this kind are called loess beds.
177.Wind Erosion. -Not only does the wind take up particles ofdust and sand and carry them from one place to another， but it uses these particles to cut and erode obstacles in its path. The artificial sand blast is in common use. In it a stream of sand is driven with great ve-locity upon an object which it is desired to etch. In nature the same kind of etching is done by the wind-blown sand.
The glasses in the windows of lighthouses along sandy coasts are sometimes so etched as to lose their transparency. Rocks exposed to the winds are carved and polished； the softer parts are worn away more rapidly than the harderparts， just as in all other forms
TREE BEING DUG UP BY THE WIND.
of erosion. In certain regions
where the prevailing winds are in one direction， one side of exposed rocks is found to be polished， while the other sides remain rough.
178.Wind Burying and Exhuming. -In exposed sandy regionswhere there are strong winds， objects which obstruct the movement of the air cause deposition of the transported sand just as obstructions in flowing water cause sediment to be deposited. And just as sand bars may be deposited by a river and then carried away again， owing to a change in the condition of the river"s load， so forests and houses in sandy regions are sometimes buried， to be uncovered again perhaps by a change in the load carried by the wind.
179.Sand Dunes. -Sand-laden wind generally deposits its burdenin mounds and ridges called sand dunes. When once a deposition pile begins， it acts as a barrier to the wind and thus causes its own further growth. In great deserts where the wind is generally from one direction these sand dunes sometimes grow to a height of several hundred feet， but usually they are not more than 20 or 30 feet high.
A FOREST ON CAPE COD.
The trees are being engulfed in wind-blown sand.
They generally have a gentle slope on the windward side and a steep slope on the leeward side. The sand is continually being swept up the windward side over the crest， thus causing the dune to move forward in the direction in which the prevailing wind blows. (Fig. 117.)Fig. 1 17.
Dunes make travel difficult， as both in climbing and descending the traveler sinks into the yielding sand. Almost no plant life can findlodgment in these shifting sand piles， so the wind continually finds loose sand on which to act， and a dune country is always a region of shifting sands. As the dunes move in the direction of the prevailing wind they sometimes invade a fertile country， so that it becomes necessary if possible to find a way to check their movement. This has been done in some places by planting certain kinds of grasses capable of growing in the sand and thus protecting the sand particles from the action of the wind.
Sand dunes are found along almost all low sandy coasts， and they render difficult the building and maintenance of roads and railroads to many beach towns.
Summary. -Besides the sculpturing of waves and rivers， two other agents of erosion are glaciers and winds. Alpine glaciers are formed by huge masses of ice and snowcrowding into mountain valleys where the snow never melts entirely. Glaciers are intersected by great cracks called crevasses and they carry accumulations of débris called moraines. Icebergs are the ends of glaciers which have broken off.
The northern part of America
QUARRYING A SAND DUNE TO MAKE BRICK.
was once covered by a huge glacier at a time which we call the Glacial Period. This glaciation has had a great effect upon the region covered. Glaciers smooth out irregularities in the surface， grind rocks， transport soil and bowlders， dam lakes， force rivers to seek new channels and on account of this create waterfalls. Thus the glaciers of the Glacial Period have had a great influence upon the conditions of life.
The winds not only blow the clouds about over the land， but they bear dust and sand with which they sculpture and erode rocks and cliffs. They also build up sand dunes and by moving them over the surface of the land sometimes destroy forests and fields.
QUESTIONS
How are glaciers formed？ Where are they found？ What do they do？
How large and how thick is the Greenland icefield？
How are icebergs formed？ Why are they dangerous？
Describe the different kinds of deposits and formations due to glaciers.