Surface material of Idaho	Surface material of northern Idaho

Details

Unit name The name of the unit Unit thickness The thickness of the unit, either < 100ft or > 100ft Minimum age The minimum age of the material Maximum age The maximum age of the material General description General description of the material

Options

Transparency Determines how much the underlying layer(s) will show through this layer. Color by material: color each unit by the type of material it represents age: color each unit by the age of the material none: leave all units transparent, except for the outline Border color Defines the color used to draw the unit outlines

Origin

This data is available directly from the USGS.

Description of data source

Abstract

Purpose

The USGS National Cooperative Geologic Mapping Program (NCGMP) works with the State geological surveys to identify priority areas for mapping of surficial materials (for example, in areas of complex and poorly understood deposits of various sediment types, where metropolitan areas are experiencing rapid growth). To help establish these priorities, a quickly prepared, modern, synoptic overview of the geology was needed. The Soller and Reheis (2004) map was made in response to that need, and provides an overview of current knowledge of the composition and distribution of surficial materials in the conterminous United States (the map covers only the conterminous U.S. because similar geologic information in digital form was not readily available for Alaska and Hawaii). Before its publication, the best available map had been a highly generalized depiction at 1:7,500,000-scale (about 120 miles to the inch), prepared for the USGS National Atlas (Hunt, C.B., 1979, Surficial geology of the conterminous United States: U.S. Geological Survey, National Atlas of the United States of America, scale 1:7,500,000).

The Soller and Reheis map was compiled at a slightly more detailed scale (about 80 miles to the inch) than Hunt's map, and used digital methods, which enabled rapid incorporation of the variety of available source maps. State-scale geologic maps from the western United States were brought directly into the map, without expending the time needed to resolve interpretive differences among them. Therefore, abrupt changes in surficial materials are indicated along many State boundaries. This of course is an artifact of the compilation technique, and a limitation on its utility. However, this approach supports the basic premise of the map -- to provide an overview of surficial materials, and to identify areas where additional work may be needed in order to resolve scientific issues that can, in turn, lead to improved mapping. The map also serves to illustrate for educational and planning purposes the general nature and distribution of the Nation's surficial materials at land surface, but does not offer information useful for local decisions because it is not intended to be used at a larger (greater detail) scale than 1:5,000,000.

Supplemental Information

In some cases, units on source maps could not be readily assigned to our classification. As an example, a geologic unit found mostly in the southeastern U.S. is a residual material that developed mostly in metamorphosed sedimentary rocks and, to a lesser extent, in sedimentary rocks. Rather than define a new map unit, we decided to classify it as "Residual materials developed in sedimentary rocks" to distinguish it from "Residual materials formed in igneous and metamorphic rocks". As another example, the source map for New York and New Jersey classified a unit as "Ice-contact deposits and glacial lake deposits -- A complex of ice-contact sand and gravel and glacial lake sediments." This unit could not readily be assigned in our classification to either a coarse- or a fine-grained unit. Therefore, we decided to assign it to a genetically-related unit, glacial till, whose texture ranges from coarse to fine. This decision certainly is imperfect, and illustrates the difficulties in reinterpreting source map information.

A further challenge is presented where map units from different source maps abut. When compiling a map from numerous published sources, many instances occur where, especially along the edges of adjoining source maps, the materials are described and mapped in different ways. Normally, these inconsistencies can be resolved by additional field mapping or through discussions with the geologists who created the source maps, and this is an especially effective approach when mapping at a relatively detailed scale. Lacking such avenues for resolution, units shown on adjacent source maps could not always be reconciled. Therefore, this map shows numerous instances where different map units meet along straight lines, commonly at state or latitude boundaries. For example, in North Dakota, a map unit extends westward where it seems to correspond to a unit from the adjacent source map. However, on that adjacent map, the surficial materials were not shown; there, it must be assumed that the bedrock is exposed and has been weathered to produce residual materials. The residual materials that are inferred from that map are classified differently than the materials described on the map to the east, hence there appears to be a sharp discontinuity in surficial materials. Because this map is an overview, essentially a "snapshot" of current knowledge that can be represented at a national scale, these inconsistencies are retained to indicate what is actually known about the materials, and to indicate where additional mapping may be beneficial.

Regarding the thickness of these surficial materials, scant information at a regional or national scale is available, except within the glaciated area. In most places, this is not a significant problem because these materials generally fall within our lower thickness category (<100 ft). However, for large expanses of alluvial and lacustrine materials (for example, in the Mississippi River Valley, the Platte River Valley, and in internally drained valleys of the western United States), thicknesses may exceed 100 ft, even where not so indicated on the map.

In most places, especially in areas not covered by glacially-deposited sediment, the uppermost material generally constitutes most (and in places, all) of the total thickness of sediment overlying bedrock. However, where sediment is shown exceeding 100 feet in thickness, the surficial material mapped at land surface does not necessarily extend downward to bedrock. In many places the uppermost material may be only a few feet thick, but the total thickness of sediment overlying bedrock is much greater. This is especially common in the glaciated area where the peat or loess shown on the map is only a thin veneer that overlies a complex package of other surficial materials which, in places, exceeds 1000 ft in thickness.