![]() California's Imperial Valley, and several other valleys within the American Southwest, exhibit this internal drainage. (See the map below.) Normally dry streams in each valley either connect to a major through-flowing river, such as the Gila or Salt rivers, or else drain into a valley's internal low spot where a salt-encrusted playa forms. This curious country consists of broad, low-elevation valleys rimmed by long, thin, parallel mountain ranges, which extend from northern Mexico across much of Arizona, California, Utah, and Nevada, northward to the southern plains of Idaho. The Sonoran Desert lies in a region of the West called the Basin and Range geologic province. ![]() The Topography of the Sonoran Desert and Adjacent Lands Some desert plants, for example, are well-adapted to soils that wouldīe toxic to other plants. Biologically-produced gases (oxygen, carbon dioxide, methane, nitrous oxide) maintain a chemically reactive atmosphere that in turn influences rates of rock weathering, the nature of sedimentary deposits, and the content of gases in the atmosphere.ĭesert soils, highly variable in their water-holding capability, salinity,Īnd alkalinity determine the kinds of plants that will survive on them. These changes may block or expedite migration for a terrestrial animal whileĪncient life affects later geologic and climatic conditions. For example, climate dictates that a river be perennial or intermittent, and that a lake expand or dry up. Migratory routes are often determined by geologic processes. Then as mountains slowly erode toĪs climates and habitats change, plant and animal species either adapt, migrate to more favorable ground, or become extinct. Sub alpine spruce-fir forests, cool enough to have supported semi-permanent ice masses on shady north slopes during the Pleistocene (the past two million years). The Sonoran Desert and nearby mountain islands exhibit nearly two miles of vertical relief, from sea-level deserts to mountaintops at 9500 feet (2900 m) that harbor Upland canyons, piedmonts, and mountaintops create new ecological niches, sites of adaptations and evolutionary change. Mountain chains appear near coastlines for various geologic reasons, setting up orographic (mountain-induced) cooling of rising moist air masses to form coastal fog deserts and rain shadow deserts on the protected sides, such as coastal Baja California and the hyper-dry Mohave Desert, respectively. The least obvious is slow continental drifting across lines of latitude or longitude, which affects circulation patterns in the oceans, storm tracks, mean temperatures, and the timing and duration of seasons. There are many ways the earth may influence a local ecosystem. Inorganic environmental changes occur, and all life forms must adapt quickly, in terms of geologic time. Continents, oceans, the climate, our atmosphere, and all life have co-evolved on this planet in a complex, interwoven web. Geology, the interpretation of earth and life history, encompasses much more than the study of sterile rock masses. Geologic Interactions with the Living Community Geology warns us not to be too literal as we imagine the history of Geology deals with continents that drift, collide and re-form, with rivers and oceans that appear and disappear, with mountain ranges whose battered remnants have been carried away and now lie buried on some other continent. ![]() But geology mocks our notion of permanence. What was here, we ask, when dinosaurs roamed, or when mountains were new? Though we may imagine different landforms, different vegetation, we probably still imagine "here" as a definite, and permanent, spot on the globe, a place with constant longitude and latitude, a place whose changes we could trace through time, perhaps by reading the record in the strata beneath our feet. We often wonder what the land we stand on was like in times past. What can 10,000 years possibly mean to us, let alone 65,000 or 70 million? Space, in geologic terms, is an equally difficult notion, though we don't often recognize the difficulty. The Geologic Origin of the Sonoran Desert
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