The Spring Mountains ECHO

What Were the Spring Mountains Like During the Last Ice Age?

By Steve Brittingham, Nevada Division of Forestry

The present geologic period we live in is the Quaternary, which is divided into the Pleistocene, covering four glacial episodes, and the present day Holocene. The latest glacial episode, the Wisconsin, reached its maximum 18,000 years ago and ended approximately 10,000 years ago. The Wisconsin glacial period produced profound effects on the desert southwest, producing lower winter temperatures of approximately 6C, decreasing evaporation, and increasing winter precipitation. The resulting vegetation communities differ in composition and

altitude ranges than previously existed. Glaciers formed in the Rocky Mountains, Sierra Nevadas, Cascades, and, to a lesser extent, the Great Basin mountain ranges. Climatic conditions of the Wisconsin ice age produced huge lakes within the Great Basin area, including Lake Lahontan, of which Walker Lake and Pyramid Lake are remnants, and Lake Bonneville, which attained a depth of 990 feet and covered an area slightly smaller than present day Lake Michigan. The Great Salt Lake of northwestern Utah is the result of evaporation of Lake Bonneville, due to Holocene warming and desiccation. The Spring Mountain Range was well watered, with most main and side canyons supporting year-round streams.

The pollen record (history) in Clark County indicates that areas now covered with desert scrub had supported pines, firs, spruce, juniper, and oak. Some presently widespread species, such as ponderosa pine, were generally restricted to small ranges in southern Arizona and New Mexico, while species with limited distribution today, such as bristlecone pine, were found from the Chihuahuan Desert into the Great Basin Desert. Desert plants, such as Apache plume and desert almond, were mixed among limber pine that grew from the lower mountain foothills, almost to the shoreline of pluvial (precipitation derived) lakes. In the Mojave Desert limber pine grew in association with Utah juniper to elevations as low as the entrance at Red Rock National Conservation Area, while bristlecone pine were found down to approximately 5000 feet elevation. Bristlecone and limber pines are presently found above 9000 feet. Pinyon pine and juniper woodlands stretched between the Spring Mountain Range and the Sheep Range. The elevational lowering of plant distributions may have been great enough to form a continuous conifer belt from the Wasatch Front (Utah) to the Sierra Nevadas, limited only by soil conditions.

Around 9500 years ago, vegetation such as creosote bush and blackbrush, adapted to the hotter and dryer climate of the Holocene, began replacing the conifers at lower elevations. In the Spring Mountains, Joshua trees and blackbrush came to dominate the foothills. New vegetation associations then formed in zones at increasing elevations. Pinyon pine and juniper formed a belt around the mountain range above the desert scrub, ponderosa pine and white fir became dominants in the mid-elevations, while limber and bristlecone pines dominated the subalpine region.

Pleistocene glaciations began with the Nebraskan glacial period approximately 1.5 million years ago. Interglacial periods ranging from 10,000 to 16,000 years separate the Nebraskan, Kansan, Illinoisan, and Wisconsin ice ages. These short interglacial periods, including the Holocene, favor the range expansion of some species such as ponderosa pines and human beings. However the fossil records indicate that species adapted to the much longer glacial periods enjoy the longest period of relatively optimal conditions. The present climate and biotic structure is just a moment in a complex of global processes and geologic time.