USING LIDAR TO IMPROVE QUATERNARY MAPPING: EXAMPLES FROM THE WALLACE 1° X 2° QUADRANGLE, IDAHO AND MONTANA, USA

The Wallace quadrangle is one of twelve 1° x 2° quadrangles in Idaho and Montana that will be integrated by the U.S. Geologic Survey’s (USGS) Northern Rocky Mountains transect (NRMT) project to produce a seamless geologic map. Geomorphic features in the Wallace quadrangle display remnants of Pleistocene glaciation related to the Flathead Lobe of the Cordilleran Ice Sheet, Glacial Lake Missoula, and multiple flood events from failed glacial dams. Previous mapping efforts of the Wallace quadrangle provide general identification and spatial extent of these Pleistocene glacial and lacustrine deposits, as well as Holocene alluvial and fluvial deposits.

For the present study, we combine previous mapping efforts with lidar modeling, such as hillshade and slope information, to refine surficial contacts and identification. Lidar coverage of both 1 m and 10 m resolutions, provided by the USGS 3D Elevation Program, helped reveal topographic features that are otherwise obscured by vegetation. Where 1 m data are available, basic identification of Quaternary units such as glacial, lacustrine, alluvium, etc., can be distinguished based on the trends of features and units can be refined where previously mapped across the quadrangle. Where 10 m data are available, existing contacts can be refined, but with less detail and confidence than in areas with 1 m data.

By using lidar data to critically assess the geomorphic expression of previously mapped Quaternary units, we have been able to significantly extend and improve the mapping of Pleistocene and Holocene units across the Wallace 1° x 2° quadrangle. Future work could test our projections with targeted field work. More broadly, our experience suggests that lidar data can be used to improve mapping of glacial and alluvial deposits elsewhere.