Geology of the Great Basin

2. The Great Basin

2 the great basin As a child, I would trace the outlines with a finger and follow the borders of the earth’s great features on maps. My fingertip could scale the Himalayas, struggling upward beside Tenzing Norgay and Sir Edmund Hillary; sail the South Pacific with the great Polynesian voyagers; explore the icy recesses of the Arctic, searching for Sir John Franklin and the crew of the Erebus. But nowhere did my finger roam a more mysterious region than the Great Basin. The Great Basin! A youthful imagination conjured visions of a vast sandy bowl, replete with rattlesnakes and alkali flats, rimmed totally by lofty mountains . Nowhere did the map depict a river or even a creek flowing out of the great sink. Everywhere the drainage lines slanted toward the interior. My finger would quiver as it walked the rimrock around one-quarter of a million square miles of the dry bowl. The basin contains almost all of the state of Nevada, half of Utah, and portions of California, Oregon, Idaho, and Wyoming. New England would be lost in one corner. The east and west boundaries were easy to trace. The snow-covered heights of the Sierra Nevada blocked the westward drainage toward the Pacific. From the Klamath area of northern California to the Mojave Desert of southern California, the great diagonal northwest-southeast Sierran wall, the highest range in the continental United States, hemmed in the Great Basin to the west. The eastern rim was as distinctly easy to finger-trace as was the western edge. Another great diagonal wall of upthrust rock—the Wasatch Mountains and the high plateaus of southern Utah— formed north-south barriers to any eastern drainages. The northern rim, less distinct, ambled east-west along the low lava- 8 The Great Basin capped ridges of the Snake River Plain of Oregon and Idaho. The tributaries of the Deschutes, John Day, Owyhee, and Snake rivers appeared in the atlas as tentacles extending southward from the vast drainage basin of the Columbia River, the colossus of the Northwest. The fluid tentacles seemed to claw at the low northern rim of the basin, pushing it southward in a gently arcing retreat into Nevada. Only along the southern rim is the boundary ragged and indistinct. Here the deep incision of the Colorado River slices westerly from the high plateaus and gnaws deeply into the edge of the basin. The master drainage of the American Southwest has deeply penetrated to the north, connecting some of the valleys of southwestern Utah and southeastern Nevada with the ocean. The deeper the Colorado cut through the shales and sands of the plateaus, the steeper is the course of its tributaries. This high gradient gives power to the streams, and they have sliced deep canyons into the southern heartland of the Great Basin. It was only later that I realized my youthful picture of the Great Basin was completely incorrect. The boundaries were accurate, but a featureless The Great Basin (from Stewart). The Great Basin 9 sand-filled bowl with lofty mountainous rims it is not. On the contrary, the floor of the bowl is broken into high snowcapped mountain ranges separated by broad elongated valleys. Although the drainages never reach the sea, it is by no means a lowland. The valley floors are generally between four and five thousand feet above sea level, and the intervening mountains tower up to thirteen thousand feet. The valley floors in the central part of the basin are higher than along its flanks. The Great Basin resembles an upside-down, broken, and cracked bowl. No surface water leaves the Great Basin except by evaporation. Biologists and anthropologists have confused the usage of the term Great Basin by utilizing other criteria to define Great Basin biota or cultures, but the definition of the geographic region is founded only on hydrology. Geologists have also inadvertently added to the confusion involving the definition of the Great Basin. We recognize the geographic and hydrologic character of the region. However, we include it as the northern portion of a much larger geologic province with a name so similar as to often cause perplexity even among its residents: the Basin and Range. This geologic territory includes virtually all the Great Basin and extends south and east through Arizona, New Mexico, and Texas, all the way into Mexico. The Basin and Range can be differentiated from its neighboring geologic regions...