GEOLOGIC CONTEXTS OF MINNESOTA’S SAND AND GRAVEL PITS

The paradox of sand and gravel (aggregate) mapping is how it differs from surficial geology mapping. With the publication of the new, statewide D-1 Surficial Geology map by the Minnesota Geological Survey (2019) a direct comparison can be made in ArcMap with a layer containing a compilation of all known Minnesota pits and quarries by using a spatial join. This allowed the first good correlation of individual pits with specific landforms, depositional environments, geological formations, and much else, on a statewide basis.

After eliminating several hundred points that erroneously plotted outside state boundaries, 21,313 pits remained, distributed in every county across the state. This includes pits of all textural types, including bedrock. Among the labeled feature types (54% of the data set) sand pits and gravel pits collectively constituted 89% of the pits.

Langer’s (2011) national aggregate summary for the USGS emphasized the predominance of the glaciofluvial category in the glaciated portion of the United States and Minnesota fits that pattern. Of the 21,313 pits, the largest category was glaciofluvial (36%), followed by glacial (32%), glaciolacustrine (14%), fluvial (8%), and smaller percentages for a total of 11 landform types. Only 3% were bedrock quarries for crushed rock aggregate.

The paradox, it turns out, is scale dependent. While glaciofluvial outwash, for example, covers many square miles and is thus satisfactorily dealt with at the 1:100,000 scale at which much county-based geologic mapping takes place, often what makes an economically viable sand and gravel deposit is site specific. Depth of overburden, thickness of deposit, depth to water table, and fraction of deleterious constituents, can vary widely over short distances, all within an otherwise seemingly homogeneous Quaternary map unit. But without the geologic insights provided by prior surficial mapping, the task of the aggregate mapper would be much more difficult.