Denver: An Archaeological History

2: Geology and Environment

c h a p t e r 2 GeoloGy and environment Although it is not perceptible to someone standing in the middle of the city itself, Denver occupies the deepest portion of a subsurface structural feature that extends from near Colorado Springs north to Wyoming (Fig. 2.1). Termed the “great trough,” the Denver Basin is part of the Colorado Piedmont Section, a division of a larger grouping of landforms known as the Great Plains Physiographic Province (Madole and Rubin 1984).The Colorado Piedmont Section encompasses much of eastern Colorado east of the mountains, and is characterized byTertiary sedimentary rocks that have been eroded by the action of the South Platte and Arkansas Rivers.Most of the Greater Denver study area lies in the Colorado Piedmont Section.The High Plains Section, a second physiographic feature of the same province, includes the portion of eastern Colorado that still retains Tertiary rocks and encompasses the Black Forest region in the southeastern part of the study area. Our study area, although technically mostly in the Great Plains, also includes the eastern foothills of the Southern Rocky Mountains Physiographic Province. This foothill zone is two to three miles (3–5km) wide (Trimble, Scott and Hansen 1984). In order to understand the environmental characteristics of the Greater Denver study area today, we must briefly examine its geological history. inception and transitions Before about two billion years ago, the record that forms the geological history of the Greater Denver area is sketchy. However, for the past half billion years, it is known that the region was sculpted by a combination of four main natural processes: mountain building; deposition and erosion by oceans, rivers and streams; volcanism; and glaciers (Fig. 2.2). The Colorado Orogeny Between about 600 and 325 million years ago, the region was a flat, featureless plain covered by several shallow, interior continental seas. About 300 million years ago a geological event termed the Colorado Orogeny (formation of mountains) uplifted the original Front Range.This ancient range was eroded by wind, water, and gravity, and the resulting thick deposit of sand and gravel became the Fountain Formation, Figure 2.1. Subregions and physical landscape of the Greater Denver area. Francine Patterson. Information compiled from Halka Chronic, Roadside Geology of Colorado (Missoula, Mont.: Mountain Press Publishing Company, 1980); Donald E.Trimble and Michael N. Machette, Geologic Map of the Greater Denver Area, Front Range Urban Corridor, Colorado (Washington, D.C.: U.S. Geological Survey, 1979; Ogden Tweto, Ogden, Geologic Map of Colorado (Washington, D.C.: U.S. Geological Survey, 1979). G e o l o G y a n d e n v i r o n m e n t | 23 colorful red sandstone monuments that form such distinctive landscape points as Red Rocks Amphitheater, Roxborough Park, and farther south the Garden of the Gods in Colorado Springs (Sullivan 1992). At the end of the Paleozoic, the sea had moved to the east of Greater Denver, and the Denver Basin itself was a coastal plain. The Age of Reptiles During the next major era, the Mesozoic, several sedimentary rock units accumulated on top of the eroded surface during a series of advancing and retreating seas.The sea that had retreated east of Denver during the Paleozoic advanced again during the Triassic age of the Mesozoic (Fig. 2.2). The Morrison Formation, from the Jurassic period, contains some of the most productive dinosaur bone beds in the world. Dinosaur tracks and bones are visible in this formation near the town of Morrison, on the western edge of the Greater Denver study area (Lockley 1990),as well as elsewhere the Rocky Mountain region.The physical remnants of camps and expeditions made in the 1870s and 1880s by scientists from as far away as the Yale Peabody Museum, to study and collect these dinosaur remains, are recorded as archaeological sites themselves. Four bone quarries are known to have been worked between 1877 and 1879: three near today’s Alameda Parkway and one above Morrison.The site near Morrison is the only one with visible remnants of quarrying into the shale; shale’s natural rather quick erosion has covered up evidence of quarrying at the other sites. Some campsites have also been identified that may be work camps associated with dinosaur bone quarrying activities (Black 1994). The Dakota (Cretaceous age) formations generally were deposited in marshy environments and thus captured the remains of living things of...