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Introduction to Surficial Materials
Geologic processes such as weathering and erosion break bedrock down into smaller particles of sediment. Sediments such as clay, silt, sand, gravel, and other loose deposits which lie on top of bedrock are grouped together in the general category of surficial materials. These materials are not soils; they are the deeper earth materials that lie between the soil zone and the underlying bedrock. Soils commonly develop by weathering of the uppermost part of these materials.
Mapping Surficial Materials
When mapping the surficial geology or the extent of sand and gravel aquifers in a quadrangle, a geologist first makes observations about the surficial materials at a network of points throughout the area. These points of observation may be auger holes, road cuts, gravel pits, stream cuts, or other places where sediments are visible. The geologist describes the materials at each location using size abbreviations as detailed in the surficial materials map explanation (PDF format). Sedimentary materials range in particle size from clay (<0.002 mm) to boulders (>256 mm or 10in). The observation points are plotted on the quadrangle and the resulting surficial materials map shows what is known about the distribution, thickness, and texture of sediments in the area.
By combining materials data with well and test hole data, seismic studies, other published information, and analysis of aerial photographs, the geologist then interprets the pattern of these materials to create a geologic map.
Uses of Materials Maps
The data shown on surficial materials maps may be used for a variety of purposes by landowners, planners, teachers, or anyone else wanting to know what lies beneath the land surface. For example, it may aid in the search for economically important deposits such as sand and gravel for aggregate or clay for bricks or pottery. Environmental issues such as the location of a suitable landfill site or the possible spread of contaminants are directly related to surficial materials information. Construction projects such as locating new roads, excavating foundations and utility lines, or siting new homes are also important uses of materials data.
Surficial materials maps are often best used in conjunction with related maps such as surficial geology maps or significant sand and gravel aquifer maps. Refer to the list of related webpages below.
The photos below are examples of the various material sizes as they are observed in the field. Click on the photo for an enlarged view to get a better sense of what the units mean. Note especially the photos at the bottom of the page. These photos show interbedded layers of materials as they may often be seen in the field. Materials in a gravel pit are rarely all a single size, and these examples show their possible complexity.
Originally published as the sidebar for the Maine Geological Survey's Surficial Materials series.
Last updated on April 23, 2012
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