Title: Geologic Map of Great Sand Dunes National Park, Colorado By Richard F. Madole, D. Paco VanSistine, and Joseph H. Romig Revision version 1.1, August 2018 U.S. Geological Survey Scientific Investigations Map 3362 ABSTRACT Geologic mapping was begun after a range fire swept the area of what is now the Great Sand Dunes National Park in April 2000. The park spans an area of 437 square kilometers (or about 169 square miles), of which 98 percent is blanketed by sediment of Quaternary age, the Holocene and Pleistocene Epochs; hence, this geologic map of the Great Sand Dunes National Park is essentially a surficial geologic map. These surficial deposits are diverse and include sediment of eolian (windblown), alluvial (stream and sheetwash deposits), palustrine (wetlands and marshes), lacustrine (lake), and mass-wasting (landslides) origin. Sediment of middle and late Holocene age, from about 8,000 years ago to the present, covers about 80 percent of the park. Fluctuations in groundwater level during Holocene time caused wetlands on the nearby lowland that bounds the park on the west to alternately expand and contract. These fluctuations controlled the stability or instability of eolian sand deposits on the downwind (eastern) side of the lowland. When groundwater level rose, playas became lakes, and wet or marshy areas called cienegas formed in many places. When the water table rose, spring-fed streams filled their channels and valley floors with sediment. Conversely, when groundwater level fell, spring-fed streams incised their valley floors, and lakes, ponds, and marshes dried up and became sources of windblown sand. Discharge in streams draining the west flank of the Sangre de Cristo Range is controlled primarily by snowmelt and flow is perennial until it reaches the mountain front, beyond which streams begin losing water at a high rate as the water soaks into the creek beds. Even streams originating in the larger drainage basins, such as Sand and Medano Creeks, generally do not extend much more than 4 km (about 2.5 miles) beyond where they exit the mountains. The Great Sand Dunes contain the tallest dunes (maximum height about 750 feet, or 230 m) in North America. These dunes cover an area of 72 square kilometers (28 square miles) and contain an estimated 10–13 billion cubic meters (2.4 to 3.1 cubic miles) of sand. The dunes accumulated in an embayment that formed where the trend of the Sangre de Cristo Range changes from southeasterly to southwesterly. They owe their exceptional height to a combination of factors including range-front geometry, topography, an abundant sand supply from the nearby basin, a complex wind regime, and the Sangre de Cristo Range, which prevents continued eastward migration of dune sand deposited by the prevailing southwesterly and westerly winds. Although the sand on the surface of the Great Sand Dunes is of late Holocene age, most of this massive sand body is a complex of deposits that accumulated episodically for more than 130,000 years. DISCLAIMERS This database, identified as SIM 3362, has been approved for release by the U.S. Geological Survey (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 USGS or the U.S. Government 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 may contain copyrighted material as noted in the text. Permission to reproduce copyrighted items for other than personal use must be secured from the copyright owner. LIST OF FILES sim3362_Readme_v1,1.txt sim3362Pamphlet_v1.1.pdf sim3362_poster_v1.1.pdf sim3362_metadata.zip sim3362_metadata.met/xml sim3362_map_supplemental_v1.1.pdf -- a map containing only the geology sim3362_gsd_shapefiles.zip directory: gsd_geol_label.shp (.cpg/.dbf/.prj/.sbn/.sbx/.shp.xml/.shx) gsd_geol_line.shp (.cpg/.dbf/.prj/.sbn/.sbx/.shp.xml/.shx) gsd_geol_point.shp (.cpg/.dbf/.prj/.sbn/.sbx/.shp.xml/.shx) gsd_geol_poly.shp (.cpg/.dbf/.prj/.sbn/.sbx/.shp.xml/.shx) gsd_NationalPark_boundary.shp (.cpg/.dbf/.prj/.sbn/.sbx/.shp.xml/.shx) sim3362_gsd_geol_gdb.zip (contains GIS data as geodatabase feature classes) Geology gsd_geol_label gsd_geol_line gsd_geol_point gas_geol_poly GDS_NAtionalPark_boundary_12_2015 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://dx.doi.org/10.3133/sim3362/ or http://pubs.er.usgs.gov/publication/sim3362 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. ArcReader may be downloaded free of charge from http://www.esri.com/software/arcgis/arcreader/download.htm for viewing additional files included in this report. Suggested citation: Madole, R.F., VanSistine, D. Paco, and Romig, J.H., 2016, Geologic Map of Great Sand Dunes National Park, Colorado (ver. 1.1, August 2018): U.S. Geological Survey Scientific Investigations Map 3362, 58 p., scale 1:35,000, https://doi.org/10.3133/sim3362.