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Chapter 2: Geology
November 1, 2007 By: Pit and Quarry Staff Pit & QuarryCrushed stone, and sand and gravel, are the main types of natural aggregate used in the United States. Aggregate is used in nearly all residential, commercial, and industrial building construction and in most public-works projects such as roads and highways, bridges, railroad beds, dams, airports, water and sewer systems, and tunnels.
The widespread use of aggregate results not only from its general availability but also from economic considerations. Aggregate of good quality commonly is available near the site of use at relatively low cost. This aggregate can essentially be obtained and used with a minimum of processing.
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However, even though crushable stone and sand and gravel resources are widely distributed throughout the United States, availability is not universal. Some areas are devoid of sand and gravel, and some potential sources of crushed stone may be lacking or covered by overburden that is too thick to allow economical surface mining.
In some areas, moreover, aggregate does not meet the physical-property requirements for certain uses, or it contains mineral constituents that react adversely when used in cement concrete. Furthermore, citizens commonly prefer that crushed stone, and sand and gravel not be mined nearby.
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Sources of aggregate, such as sand and gravel and rock for crushed stone, were formed by geologic processes. Volcanoes, glaciers, wind, rivers and seas formed the shape and character of rock materials over millions of years. The gravel used today may have been deposited thousands of years ago—just yesterday geologically. Hard, dense limestone may have been deposited as a limy ooze hundreds of millions of years ago. When an aggregate supply is required, geological investigations can determine the location, distribution, and nature of potential aggregate in an area.
Sand-and-gravel deposits are products of erosion of bedrock and surface materials and the subsequent transport, abrasion, and deposition of the particles. The principal geologic agent that affects the distribution of deposits of sand and gravel is water. Consequently, gravel is widely distributed and abundant in glaciated areas, in alluvial basins, and in, adjacent to, or near rivers and streams. Windblown deposits generally are fine grained and rarely used for aggregate.
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Sand and gravel deposited by rivers or streams is widely distributed throughout the United States as stream-channel or terrace deposits. In hilly or mountainous areas, bedrock is chemically and physically weathered and is progressively broken into smaller and smaller particles. Chemically less resistant minerals are dissolved or altered into clay minerals; the more resistant minerals remain as rock fragments.
Depending on the composition and structure of the bedrock and on the climate, land cover, and topography, the remaining soils may range in thickness from almost nothing to many tens of feet, and may range in composition from nearly all clay, through mixtures of clay, silt, sand, and gravel, to nearly all sand and gravel, to rubble.
 Production and uses of aggregate |
Gravity and sheetwash move some of this material downslope, where it forms a deposit called colluvium. Eventually, the colluvium is moved into valleys of relatively high gradient streams. In the stream channels, rock fragments are subjected to abrasion, rounding, and sorting. The stream-transported material is deposited in channels and on floodplains and consists of sand and gravel in some areas and silt and clay in others.
Erosion can alter an already established floodplain. If the river or stream incises its channel, the older channel and floodplain deposits are preserved as terraces. Repeated downcutting can result in the formation of a series of terraces or terrace remnants.
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