BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 55, PP. 77-90, 2 PLS., 7 FIGS. JANUARY 1, 1944 STRUCTURE AND PETROLOGY OF SPECIMEN MOUNTAIN, COLORADO BY ERNEST E. WAHLSTROM CONTENTS Page Abstract........................................................................................................................................................................ 77 Introduction............................................................................................................................................................... 78 General statement............................................................................................................................................ 78 Location.............................................................................................................................................................. 78 Topography and physiography.................................................................................................................... 79 Methods of mapping....................................................................................................................................... 79 Acknowledgments............................................................................................................................................. 79 General geology......................................................................................................................................................... 79 Structure...................................................................................................................................................................... 80 Faulting and folding........................................................................................................................................ 80 Structural history............................................................................................................................................. 80 Regional relationships.............................................................................................................................................. 82 Petrography................................................................................................................................................................ 84 General statement............................................................................................................................................ 84 Basal flows.......................................................................................................................................................... 84 Pyroclastics........................................................................................................................................................ 86 Upper flows........................................................................................................................................................ 86 Plug rocks........................................................................................................................................................... 87 Differentiation........................................................................................................................................................... 88 Pneumatolytic and hydrothermal activity........................................................................................................ 89 Age................................................................................................................................................................................ 89 References cited........................................................................................................................................................ 89 ILLUSTRATIONS Figure Page 1. Outline map of north-central Colorado................................................................................................... 78 2. Geologic map of Specimen Mountain area............................................................................................. 81 3. Geologic cross sections of Specimen Mountain..................................................................................... 82 4. Idealized cross sections................................................................................................................................. 83 5. Geologic map showing relationship of flow at Iceberg Lake to Specimen Mountain................ 84 6. Larsen plots of Specimen Mountain rocks............................................................................................. 88 7. Triangular diagram....................................................................................................................................... 88 Plate Facing page 1. West side of Specimen Mountain......................................................................................................... 80 2. Volcanic rocks at Specimen Mountain................................................................................................ 81 ABSTRACT Specimen Mountain, an eroded Cenozoic volcano in the northwestern corner of Rocky Mountain National Park, Colorado, is flanked on the south and west by an extensive series of faulted and folded lava flows and pyroclastics. Regional studies indicate that the erupted materials spread 77 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/55/1/77/3431269/BUL55_1-0077.pdf by guest on 29 September 2021 78 WAHLSTROM— SPECIMEN MOUNTAIN, COLORADO laterally for several miles in all directions from a single central conduit and buried a prevolcanic . surface of considerable relief. The volcanic rocks, as shown by analyses, are quartz latite and silicic rhyolite. Quartz latite flows are chilled samples of a magma which by gravitative differentiation gave rise to silicic rhyolite occurring as flows and plug rocks. Thick beds of ash, agglomerate, and breccia attest to inter­ mittent explosions of great magnitude. Inconclusive evidence suggests a Miocene or Pliocene age for the volcanics. F ig u r e 1. Outline map of north-central Colorado Showing location of Specimen Mountain area. INTRODUCTION GENERAL STATEMENT The study of Specimen Mountain was undertaken in collaboration with the National Park Service in an effort to unravel the geological history of this prominent and often-visited landmark. The project is one of several leading ultimately to the reconstruction of the geologic and physiographic history of the whole of Rocky Mountain National Park. This paper summarizes results of geologic and petrologic investigations begun in the summer of 1941 and continued intermittently through the winter of 1942-1943. LOCATION Specimen Mountain straddles the Continental Divide at the crest of the Front Range of Colorado (Fig. 1). Its western slopes drain into the headwaters of the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/55/1/77/3431269/BUL55_1-0077.pdf by guest on 29 September 2021 INTRODUCTION 79 Colorado River in the northwestern corner of Rocky Mountain National Park. The Cache la Poudre River, a tributary of the South Platte River, drains the southern and eastern slopes. The mountain is easily reached by the Trail Ridge Road from either Grand Lake or Estes Park. A trail starting at Poudre Lake leads to the top of Specimen Mountain. TOPOGRAPHY AND PHYSIOGRAPHY The elevation of the summit of Specimen Mountain is 12,482 feet. Slopes on the northwest and south sides of the mountain drop steeply to the floors of canyons 2500 to 3000 feet below the summit. The area surrounding Specimen Mountain is exceedingly rugged and displays all the depositional and erosional features of intense alpine glaciation. Specimen Mountain itself is glaciated and is deeply embayed on its northern and western slopes by cliff-walled cirques, which are rapidly filling be­ cause of landsliding in the rapidly disintegrating volcanic rocks. Evidences of frost action and solifluxion are abundant on the upper exposed slopes. Polygonal rock patterns and rock streams are locally conspicuous. In the pre-Cambrian rocks near Specimen Mountain much of the surface has a “grain” which reflects the gneissoid structure of the underlying rocks. Lenticular masses of pegmatite and granite strike northeast and, in general, stand out in ridges formed by more rapid erosion of adjacent layers of schist and gneiss. METHODS OF MAPPING Geologic mapping was done on aerial photographs. A composite geologic map was made from the various photographs, and errors due to tilting and perspective were ad­ justed by use of a triangulation net surveyed with a plane table and alidade. The geologic map was then superimposed on a photographic enlargement of a portion of the United States Geological Survey’s topographic map of the Rocky Mountain National Park Quadrangle. Because of the inaccuracy of the enlarged topographic map only the 500-foot contours are shown in the finished maps. ACKNOWLEDGMENTS The writer wishes to express his appreciation of the assistance given in the field by Mr. Olwin Woodbury. The work was considerably expedited by the helpful co­ operation of several officials of Rocky Mountain National Park. The writer is es­ pecially indebted to Mr. David H. Canfield, Park Superintendent, Mr. Raymond Gregg, Park Naturalist, and Dr. Carroll H. Wegemann, Regional Geologist, for many courtesies. H. E. Rothrock, Assistant Chief of the Naturalist Division of