Frontiers in Colorado Paleoindian Archaeology: From the Dent Site to the Rocky Mountains

1. Late Quaternary Prehistoric Environments of the Colorado Front Range

11 James P. Doerner C h a p t e r o n e Late Quaternary Prehistoric Environments of the Colorado Front Range This chapter examines the prehistoric environments of the Colorado Front Range during the past 25,000 years, the interval encompassing the most recent glacial-interglacial cycle (Porter 1983). This interval is generally referred to as the late Quaternary Period and is a critical time in both human and earth history. It was during this time that humans first arrived in North America, large-scale extinctions of Pleistocene mammals occurred, and boreal vegetation consisting mainly of tundra plants and spruce woodlands covered large areas of North America south of the continental ice sheets (Holloway and Bryant 1985). The interval also includes the termination of the Pleistocene and establishment of a new post-glacial macroclimate during the Holocene. These new environmental conditions meant that old plant associations and distributions had to adjust to remain in equilibrium with the changing environment (Baker 1983). As glacial James P. Doerner 12 1.1. Map showing major physical features of Colorado. Shaded area shows the generalized location of the Colorado Front Range in north-central Colorado. conditions ameliorated and alpine glaciers melted away, vegetation zones apparently responded by advancing upslope from the plains and the lower montane zones to reclaim the higher elevations (Elias 1988). Environmental changes during the late Quaternary have been substantial . Climate is probably the most important environmental factor, controlling growth and decay of ice sheets, distributions of flora and fauna (including human populations), development of soils, and erosion and sedimentation processes on the landscape (Wright 1983). The reconstruction of past environments is essential for interpreting past human occupations and cultural landscape modeling. The remainder of this chapter is divided into four sections: (1) a description of the regional setting, (2) a review of climate reconstructions based on glacial chronologies, (3) an overview of paleoecological studies derived from pollen and fossil beetle investigations, and (4) a summary of late Quaternary climate change in the Colorado Front Range. REGIONAL SETTING The Colorado Front Range (Figure 1.1) is made up of the easternmost mountain ranges that comprise the Southern Rocky Mountain physiographic province Late Quaternary Prehistoric Environments of the Colorado Front Range 13 1.2. Map showing major air mass trajectories into the Colorado Front Range. (Hunt 1967). The Front Range is about 300 km long, extending from the Arkansas River in the south to the Colorado-Wyoming border in the north (Short 1985). The eastern margin of the Front Range is clearly defined by steeply dipping sedimentary layers that mark the contact with the Great Plains. Along the western margin, the Front Range is bordered by other mountain ranges and intermontane basins (Marr 1967). The topography throughout most of the area is rugged and steep, with broad glaciated valleys prominent above 2,800 m (Veblen and Lorenz 1986). The modern climate is classified as highland continental, with short cool summers, long cold winters, and relatively dry conditions throughout the year (Griffiths and Rubright 1983). Dramatic changes in average temperature and precipitation occur with increased elevation (Barry 1972). Marr (1967) notes that the area is subject to extreme changes in atmospheric conditions, which can occur from hour to hour, day to day, season to season, and year to year. Climate is also strongly influenced by its position relative to prevailing air mass trajectories (Figure 1.2). During winter months, westerly circulation brings cool, moist Pacific air masses to the mountains. This can produce heavy snowfalls for the mountains west of the Continental Divide, while the east slope of the divide experiences relatively little precipitation (Veblen and Lorenz 1991). The western slope has a wet season during winter months when the westerlies are the strongest, while the eastern slope is relatively dry during winter (Rink and Kiladis 1986). The Front Range receives most of its annual precipitation from storm systems originating in the Gulf of Mexico (Marr 1967). Maximum precipitation on the east slope usually occurs during the spring (Rink and Kaladis 1986). A secondary peak James P. Doerner 14 occurs in July and August, reflecting the influence of summer convective storms (Barry 1972). Elevation can also influence the timing of the precipitation peak for east slope locations in the Front Range (Barry 1973). Elevations at 3,000 m have a spring precipitation maximum, whereas areas above 3,750 m have a winter precipitation peak (Barry 1973). Elevations in the...