PRELIMINARY MAP OF SURFICIAL MATERIALS IN THE CONTERMINOUS UNITED STATES

Because surficial materials are important to society, it is important to understand their composition and distribution. The USGS National Cooperative Geologic Mapping Program is expanding its emphasis on mapping surficial materials. To facilitate the identification of priority areas for later detailed mapping (such as metropolitan areas experiencing rapid growth), we used a simple earth materials classification to compile quickly a preliminary digital map of surficial deposits of the conterminous U.S.

We used digital source maps that were statewide or larger in area that either showed surficial materials or could be interpreted to derive such information. For roughly the eastern half of the U.S., we used a version of the Quaternary Geologic Atlas of the United States (scale 1:1,000,000) that we simplified by combining units with similar properties into broader materials categories (e.g., alluvium, eolian sand) and recompiled to 1:2,500,000 scale. For the western half of the Nation, we used published statewide geologic maps (mostly 1:500,000 scale). Although some of these show complex surficial units, most emphasize the bedrock geology. In each digital map file, surficial geologic units were assigned to the appropriate category in our classification, and bedrock units were assigned to the surficial materials category (e.g., residuum type) most appropriate to their rock composition. These maps were merged into a single digital file and then supplemented with information from various other maps, such as those displaying extents of pluvial lakes and eolian sand deposits.

Our rapid overview approach has advantages and disadvantages; the map was generated relatively quickly and easily, but it does not provide a well-compiled and coherent view of the deposits. For example, no effort was made to resolve interpretive differences among state-scale geologic maps in the western U.S. nor to merge these sources with the coherent Quaternary Atlas source maps used for the eastern U.S. Abrupt changes in surficial materials along many state boundaries are an artifact of our mapping, and a limitation on its utility. However, this approach serves to provide an overview of surficial materials, and to identify areas where additional work is needed in order to resolve scientific issues that can, in turn, lead to improved mapping.