Geology: The Science of the Earth's Crust. William J. Miller

débris-laden streams emerging from the margins of glaciers, the water sometimes rising as great fountains because of the pressure. Such deposits are now actually in process of formation along the edge of the great Malaspina Glacier of Alaska. Kames are commonly associated with terminal and recessional moraines. “Eskers” are similar except that they are long winding low ridges of stratified material deposited by débris-laden streams, probably in longitudinal fissures in the ice near its margin. (See Plate 20.)

      Glacial bowlders, or “erratics” are blocks of rock or bowlders left strewn over the country during the melting of the ice. They vary in size from small pebbles to those of many tons of weight, and most of them were derived from ledges of relatively hard, resistant rocks. (See Plate 20.) Erratics have very commonly been carried a few miles from their parent ledges, while more rarely they have traveled even hundreds of miles. They are extremely abundant in New York and New England, many occurring even high up on the mountains. In some cases erratics of ten or more tons' weight have been left in such remarkably balanced positions on bedrock that a child can cause one of them to swing back and forth slightly. Such a bowlder is literally a “rocking stone.” In the Adirondack Mountains the writer recently observed a rounded erratic of very hard rock fourteen feet in diameter resting in a very remarkably balanced position on top of another large round glacial bowlder.

      CHAPTER VI

      THE ACTION OF WIND

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      ONLY during the last quarter of a century have geologists come to properly appreciate the really important geological work of the wind. One reason for this is the fact that people live mostly in humid regions where the soils are largely effectually protected against wind action by the vegetation. But even in such regions, wind action is by no means negligible. One has but to observe the great clouds of dust raised by strong wind from freshly cultivated fields during a little dry weather in the late spring. Much of this dust is carried considerable distances, often miles, and in some cases young crops are injured by removal of soil from around the roots, while in other cases young plants are buried by deposition of the wind-blown material over them. In humid regions, the action of the wind is perhaps most strikingly exhibited along and near shores of sea and lakes, where loose dry sands are picked up and transported in great quantities, often depositing them as sand dunes, which may form groups of hills covering considerable areas. Very conspicuous examples are the sand dunes of Dune Park in northern Indiana, and the dunes along the coast of New Jersey.

      But the action of wind is most strikingly effective in desert and semiarid regions. The importance of the work of wind is made more impressive when we realize that about one-fifth of the land of the earth is desert.

      In deserts some of the ordinary agents of weathering and erosion are either absent or notably reduced in effectiveness. Thus, stream action is, in general, reduced to a minimum; weathering effects due to moisture in the air are notably reduced, and either frost action, or wedge work of ice, is relatively unimportant due to lack of water. Change of temperature between night and day is, however, unusually important as a process whereby rocks are broken up due to relatively rapid expansion and contraction in deserts because such temperature changes are exceptionally great, and rocks and soils are almost everywhere directly exposed, being free from vegetation.

      The finer grained materials, especially sand grains, in deserts are picked up by the wind and driven, often with great velocity, against barren rock ledges and large and small rock fragments. By this process (corrasion) the rocks are worn and often polished by the materials blown against them. The principle is that of the artificial sand-blast, used in etching glass, or cleaning and polishing building and decorative stones. Under favorable conditions wind-driven sand accomplishes noticeable erosion in a surprisingly short time. Thus, in a hard wind storm, a plate glass window in a lighthouse on Cape Cod was worn to opaqueness, while in a few weeks or months the directly exposed window glass may there be worn through.

      The great erosive effect of wind-driven sand is relatively close to the ground because the larger and heavier fragments are not lifted to very considerable heights. For this reason ordinary telegraph poles are difficult to maintain in desert regions because, unless they are specially protected, they are soon cut down by sand swept against their bases. In the desert regions of our Southwestern States cliffs rising above the general level of the country are often undercut by wind erosion, sometimes with the development of large caverns. (See Plate 1.) Even the high portions of great ledges are there more or less fantastically sculptured by wind erosion, the softer portions being more deeply cut into than the harder. The famous sphinx of Egypt has been notably roughened by action of this kind.

      The enormous power of high winds to transport rock material in desert regions is strikingly illustrated by the great sand storms of the Sahara Desert, where sand and dust, forming clouds with cubic miles of volume, sweep for many miles across the country. Some one has estimated that every cubic mile of air in such a storm contains more than 100,000 tons of rock material. It is said that an army of 50,000 men under Cambyses was buried under the sands of a storm in the desert of northern Africa.

      Dust from some of these storms is known to be driven hundreds of miles out over the Atlantic Ocean, there to settle in the sea. In mountainous desert regions, like the Great Basin of our Western States, the general tendency is for the rock materials wind-eroded from the mountains to be carried into the intermontane basins or valleys. Some basins of this sort are believed to contain depths of 1,000 to 2,000 feet of wind-blown material.

      A special kind of wind-blown material called “loess,” is a sort of fine-grained yellow, or brown loam which, though relatively unconsolidated, has a remarkable property of standing out as high steep cliffs or bluffs along the banks of streams. Many thousands of square miles of northern China are covered with loess. Among many other regions, thousands of square miles of parts of the States of Iowa, Nebraska, and Kansas are covered with loess, which, in this case, is believed to be fine material gathered by winds from the region just after the retreat of one of the ice sheets of the great Ice Age, when there was very little vegetation to hold down the loose soils of glacial origin.

      Much as snowdrifts are formed, so, in many places, the wind-driven sands are built up into sand hills or so-called “dunes.” Dunes are very common in many places, as for example, along our middle Atlantic coast; in Dune Park of northern Indiana; and in the great arid and semiarid regions of the Western States. Where there is a distinctly prevailing direction of wind, the sand is blown to the leeward side from the windward side, and the dunes are caused to migrate in the direction of the wind. The burial and destruction of forests, and the uncovering of the dead trees is not uncommonly caused by migration of sand dunes, all stages of this phenomenon being well exhibited in Dune Park, Indiana. The rate of dune migration is very variable, but study in a number of places has shown a rate of from a few feet to more than 100 feet per year. Arable lands, buildings, and even towns have been encroached upon and buried under drifting sand. An interesting example is a church in the village of Kunzen, on the Baltic seashore which, in a period of sixty years, became completely buried under a dune and then completely uncovered by migration of the dune. Much destruction has been wrought by shifting sands on the Bay of Biscay, where farms and even villages have been overwhelmed. The ruins of the ancient cities of Babylon and Nineveh are buried mostly under wind-blown sand and dust. There is good reason to believe that the climate of central and western Asia is now notably drier than it was a few thousand years ago, and this may help to explain the burial of many old cities and villages there under wind-blown deposits.

      CHAPTER VII

      INSTABILITY OF THE EARTH’S CRUST

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      THE crust of the earth is unstable. To the modern student of geology the old notion of a “terra firma” is outworn. The idea of an unshakable, immovable earth could never have emanated from the inhabitants of an earthquake country. In general we may recognize two types of crustal movements—slow and sudden. To most people the sudden movements accompanied by earthquakes are more significant and impressive because they are more localized and evident, and often accompanied by destruction of property, or quick, though minor, changes in the landscape. But movements which take place slowly and quietly are often of far greater