Geologic map of the Masters 7.5’ quadrangle, Weld and Morgan Counties, Colorado by Margaret E. Berry, Janet L. Slate, James B. Paces, Paul R. Hanson and Theodore R. Brandt 2015 Scientific Investigations Map 3344 ABSTRACT: The Masters 7.5’ quadrangle is located along the South Platte River corridor on the semiarid plains of eastern Colorado, and contains surficial deposits that record alluvial, eolian, and hillslope processes that have operated in concert with environmental changes from Pleistocene to present time. The South Platte River, originating high in the Colorado Front Range, has played a major role in shaping the surficial geology of the quadrangle, which is situated downstream of where the last of the major headwater tributaries (St. Vrain, Big Thompson, and Cache la Poudre) join the river. Recurrent glaciation (and deglaciation) of basin headwaters affected river discharge and sediment supply far downstream, influencing deposition of alluvium and terrace formation in the Masters quadrangle. Kiowa and Bijou Creeks, unglaciated tributaries originating in the Colorado Piedmont east of the Front Range and joining the South Platte River just downstream of the Masters quadrangle, also have played a major role by periodically delivering large volumes of sediment to the river during flood events, which may have temporarily dammed the river. Eolian sand deposits of the Greeley (north of river) and Fort Morgan (south of river) dune fields cover much of the quadrangle and record past episodes of sand mobilization during times of prolonged drought. With the onset of irrigation and damming during historical times, the South Platte River has changed from a broad, shallow sandy braided river with highly seasonal discharge to a much narrower, deeper river with braided-meandering transition morphology and more uniform discharge. Along the reach of river in the Masters quadrangle, the river has incised into Upper Cretaceous Pierre Shale, which, although buried by alluvial deposits here, is locally exposed downstream along the South Platte River bluff near the Bijou Creek confluence, in some of the larger draws, and along Wildcat Creek. DISCLAIMERS: This database, identified as SIM3344, 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://dx.doi.org/10.3133/sim3344/ ArcInfo export files, ArcView shapefiles, and an ArcGIS geodatabase may be extracted from a zip-compressed file (SIM3344_GIS.zip). In addition, an Adobe Portable Document Format (PDF) file (SIM3344 _map.pdf) can be downloaded, from which paper copies may be printed. The database contact is: Theodore R. Brandt 303-236-1901 tbrandt@usgs.gov U.S. Geological Survey Box 25046, Mail Stop 980 Denver, CO 80225 The data were compiled from field mapping, interpretation of NAIP imagery, interpretation of aerial photographs, and interpretation of digital soil data. Daniel R. Muhs (U.S. Geological Survey), and Ralph R. Shroba (U.S. Geological Survey), reviewed the geologic map and accompanying text. This geospatial database consists of ArcInfo export files (.e00), ArcView shapefiles, and ArcGIS geodatabase files contained in a zip archive. See the list below. Please refer to the file SIM3344.met for detailed metadata documentation for this geospatial database. List of files: 00ReadMe.txt SIM3344_appendix.pdf SIM3344_map.pdf SIM3344_map_geo.pdf (georeferenced pdf for users' convenience) SIM3344.met mastbasr.tif mastbasr.tfw SIM3344_GIS.zip: e00 directory: mastgeo.e00 maxc.e00 macrests.e00 mathupts.e00 wpgcmykg.shd.e00 geol_sfo.lin.e00 geoscamp2.mrk.e00 import.aml shapefiles directory: mastgeo.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) polygon mastgeol.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line macrests.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line maxc.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line mathupts.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) point SIM3344_GIS.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://dx.doi.org/10.3133/sim3344/ 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.