National Park Service U.S. Department of the Interior Natural Resource Program Center Rainbow Bridge National Monument Geologic Resources Inventory Report Natural Resource Report NPS/NRPC/GRD/NRR—2009/131 THTHIISS PAPAGE:GE: The view from Cummings Mesa displays the stunning geologic landscape of Rainbow Bridge National Monument. Rainbow Bridge, carved out of Navajo Sandstone, is in the lower center of the image. NPS Photo. ON THE COVER: A more conventional view of Rainbow Bridge from the main trail. NPS Photo by Jonathan Parker. Rainbow Bridge National Monument Geologic Resources Inventory Report Natural Resource Report NPS/NRPC/GRD/NRR—2009/131 Geologic Resources Division Natural Resource Program Center P.O. Box 25287 Denver, Colorado 80225 August 2009 U.S. Department of the Interior National Park Service Natural Resource Program Center Denver, Colorado The Natural Resource Publication series addresses natural resource topics that are of interest and applicability to a broad readership in the National Park Service and to others in the management of natural resources, including the scientific community, the public, and the NPS conservation and environmental constituencies. Manuscripts are peer-reviewed to ensure that the information is scientifically credible, technically accurate, appropriately written for the intended audience, and is designed and published in a professional manner. Natural Resource Reports are the designated medium for disseminating high priority, current natural resource management information with managerial application. The series targets a general, diverse audience, and may contain NPS policy considerations or address sensitive issues of management applicability. Examples of the diverse array of reports published in this series include vital signs monitoring plans; "how to" resource management papers; proceedings of resource management workshops or conferences; annual reports of resource programs or divisions of the Natural Resource Program Center; resource action plans; fact sheets; and regularly-published newsletters. Views, statements, findings, conclusions, recommendations and data in this report are solely those of the author(s) and do not necessarily reflect views and policies of the U.S. Department of the Interior, National Park Service. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the National Park Service. Printed copies of reports in these series may be produced in a limited quantity and they are only available as long as the supply lasts. This report is also available online from the Geologic Resources Inventory website (http://www.nature.nps.gov/geology/inventory/gre_publications) and the Natural Resource Publication Management website (http://www.nature.nps.gov/publications/NRPM/index.cfm) or by sending a request to the address on the back cover. Please cite this publication as: Graham, J. 2009. Rainbow Bridge National Monument Geologic Resources Inventory Report. Natural Resource Report NPS/NRPC/GRD/NRR—2009/131. National Park Service, Denver, Colorado. NPS 118/100151, August 2009 ii Contents Figures .......................................................................................................................... iv Executive Summary ...................................................................................................... v Introduction ................................................................................................................... 1 Purpose of the Geologic Resources Inventory ........................................................................................................... 1 General Information and Regional Location ............................................................................................................... 1 General Geology ........................................................................................................................................................ 1 Park History ................................................................................................................................................................ 2 Geologic Issues ............................................................................................................. 5 Rainbow Bridge Stability ............................................................................................................................................. 5 Maintaining the “Sacred Integrity” of Rainbow Bridge ................................................................................................. 5 Rockfalls in the Navajo Sandstone ............................................................................................................................. 5 Other Issues ............................................................................................................................................................... 6 Geologic Features and Processes ............................................................................. 10 Rainbow Bridge and Bridge Creek ........................................................................................................................... 10 Sedimentary Features .............................................................................................................................................. 10 Dinosaur Tracks ....................................................................................................................................................... 11 “Stonepecker holes” (Tafoni) .................................................................................................................................... 11 Alcoves and Hanging Gardens ................................................................................................................................. 11 Wadis and Oases ..................................................................................................................................................... 11 Navajo Mountain ....................................................................................................................................................... 12 Joints ........................................................................................................................................................................ 12 Anticlines and Synclines ........................................................................................................................................... 12 Desert Varnish .......................................................................................................................................................... 12 High-Level Terrace Deposits .................................................................................................................................... 13 Low-Level Terrace Deposits ..................................................................................................................................... 13 Talus and Colluvium ................................................................................................................................................. 13 Petroglyphs and Historic Graffiti ............................................................................................................................... 13 Map Unit Properties .................................................................................................... 16 Geologic History.......................................................................................................... 19 Jurassic Period (199.6–145.5 Ma) ............................................................................................................................ 19 Cretaceous Period (145.5–65.5 Ma) ......................................................................................................................... 20 Tertiary Period (65.5–1.8 Ma) ................................................................................................................................... 20 Quaternary Period (1.81 Ma–present) ...................................................................................................................... 21 Glossary ....................................................................................................................... 28 References ................................................................................................................... 31 Appendix A: Geologic Map Graphic .......................................................................... 33 Appendix B: Scoping Summary ................................................................................. 35 Attachment 1: Geologic Resources Inventory Products CD RABR Geologic Resources Inventory Report iii Figures Figure 1. Location map of Rainbow Bridge National Monument .................................................................................... 3 Figure 2. Rainbow Bridge prior to the construction of the Glen Canyon Dam ................................................................ 4 Figure 3. Stratigraphic column for Rainbow Bridge National Monument ........................................................................ 4 Figure 4. A tour boat on Lake Powell enters (Rainbow) Bridge Canyon ........................................................................ 7 Figure 5. A 1983 rockfall in the Navajo Sandstone within Rainbow Bridge National Monument ...................................