Surficial Geologic Map of the Red Rock Lakes Area, Southwest Montana By Kenneth L. Pierce, Tara L. Chesley-Preston, and Richard L. Sojdat 2014 Open-File Report 2014-1157 ABSTRACT: The Centennial Valley and Centennial Range continue to be formed by ongoing displacement on the Centennial fault. The dominant fault movement is downward, creating space in the valley for lakes and the deposition of sediment. The Centennial Valley originally drained to the northeast through a canyon now represented by a chain of lakes starting with Elk Lake. Subsequently, large landslides blocked and dammed the drainage, which created Lake Centennial, in the Centennial Valley. Sediments deposited in this late Pleistocene lake underlie much of the valley floor and rest on permeable sand and gravel deposited when the valley drained to the northeast. Cold Pleistocene climates enhanced colluvial supply of gravelly sediment to mountain streams and high peak flows carried gravelly sediment into the valley. There, the lower gradient of the streams resulted in deposition of alluvial fans peripheral to Lake Centennial as the lake lowered through time to the level of the two present lakes. Pleistocene glaciers formed in the high Centennial Range, built glacial moraines, and also supplied glacial outwash to the alluvial fans. Winds from the west and south blew sand to the northeast side of the valley building up high dunes. The central part of the map area is flat, sloping to the west by only 0.6 meters in 13 kilometers (2 feet in 8 miles) to form a watery lowland. This lowland contains Upper and Lower Red Rock Lakes, many ponds, and peat lands inside the "water plane," above which are somewhat steeper slopes. The permeable sands and gravels beneath Lake Centennial sediments provide a path for groundwater recharged from the adjacent uplands. This groundwater leaks upward through Lake Centennial sediments and sustains wetland vegetation into late summer. Upper and Lower Red Rock Lakes are formed by alluvial-fan dams. Alluvial fans converge from both the south and the north to form outlet thresholds that dam the two shallow lakes upstream. The surficial geology aids in understanding how the landscapes in and around the Red Rock Lakes Wildlife Refuge were formed and how they transmit water. This report uses metric units except for altitudes that are also given in feet because contours on the base map are in feet and the reader would have to convert from metric units to feet to understand the map relationships. DISCLAIMERS: This database, identified as OFR 2014-1157, has been approved for release and publication by the Director of the USGS. Although this database has been subjected to rigorous review and is substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, it is released on condition that neither the USGS nor the United States Government may be held liable for any damages resulting from its authorized or unauthorized use. Although these data have been processed successfully on a computer system at the U.S. Geological Survey, no warranty, expressed or implied is made regarding the display or utility of the data on any other system, or for general or scientific purposes, nor shall the act of distribution constitute such warranty. The U.S. Geological Survey shall not be held liable for improper or incorrect use of the data described and/or contained herein. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey. Although this information product, for the most part, is in the public domain, it also contains copyrighted material as noted in the text. Permission to reproduce copyrighted items for other than personal use must be secured from the copyright owner. DATA FILES: The database can be downloaded via the Web from http://pubs.usgs.gov/of/2014/1157/ ArcView shapefiles, and an ArcGIS geodatabase may be extracted from a zip-compressed file (OFR2014-1157_GIS.zip). In addition, an Adobe Portable Document Format (PDF) file of the map (ofr2014-1157.pdf) and pamphlet (ofr2014-1157_pamphlet.pdf) can be downloaded, from which paper copies may be printed. The database contact is: Tara Chesley-Preston Spatial Analyst Research Associate with the Montana Institute on Ecosystems at MSU USGS-NOROCK 2327 University Way, Suite 2 Bozeman, MT 59715 Phone 406-994-7158 Fax 406 994-6556 This geospatial database consists of ArcView shapefiles, and ArcGIS geodatabase files contained in a zip archive. See the list below. Please refer to the file OFR2014-1157_Metadata for detailed metadata documentation for this geospatial database. List of files: 00ReadMe.txt ofr2014-1157_pamphlet.pdf ofr2014-1157.pdf (map) OFR2014-1157_Metadata.txt rrl_topo.tif rrl_topo.tfw OFR2014-1157_GIS.zip: shapefiles directory: ConicalDepression.shp ExposedorAuger_Section.shp GeologicFeatures.shp Lakes_Ponds.shp RRLGeologicUnitLines.shp RRLGeologicUnits.shp .gdb directory (contains GIS data as geodatabase feature classes) HOW TO OBTAIN THE DIGITAL FILES: The digital files constituting the geologic map database of this report can be obtained via the Internet from the U.S. Geological Survey publications website. Go to the web page at http://pubs.usgs.gov/of/2014/1157/ and follow the directions to download the files. The main product is a Portable Document Format (.pdf) map, which requires Adobe Acrobat for viewing. Acrobat software runs on a variety of systems, and is available for download free of charge from Adobe at http://www.adobe.com.