Question of the Day - 19 September 2022

Once in a blue moon, we get a question about Nevada geology. We've answered it twice, the first time as far back as 2006, but obviously, in geological time, that's the blink of an eye. Which is to say, nothing has changed since, so we rerun it here without the need for any updating. 

Half a billion (or so) years ago, Nevada rested underwater. At least twice during the long Paleozoic Era (340 million years), violent and titanic episodes of uplift raised the ocean floor, drained the sea, and left towering mesas and alluvial plains. Over the next 160 million years, cataclysmic extinctions, volcanism, and climatic crises, punctuated by long periods of erosion, continually altered Nevada, several times obliterating its life and landscape.

Seventeen million years ago, today’s familiar basins and ranges were created by the colossal jostle of tectonic forces. More than 250 separate mountain ranges are currently named in Nevada. Ninety percent of them are oriented northeast-southwest and on a relief map they look like a herd of earthworms marching toward Mexico. The southwest-trending cavalcade of ranges, however, jams up at a southeast-trending dead end at the northern edge of Las Vegas Valley. Geologists refer to this phenomenon (apparently without irony) as the Las Vegas Zone of Deformation.

Beginning nearly two million years ago, four great ice ages advanced into and retreated from history. Nevada’s Basin and Range Physiographic Province, which had been shuffled by earthquakes, tilted by crustal adjustments, and whittled by erosion, was now alternately drowned, drained, and ground down by glaciers. By the end of the last ice age, 12,000 years ago, Las Vegas Valley (for a minor QoD controversy over the use of the term "Las Vegas Valley," see the QoD on April 25, 2014) had finally taken the shape it retains today: relatively long and flat, cutting an 18- by 26-mile diagonal across what would become Clark County, near the southern tip of the state.

Over the millennia, alluvial fans of stone, gravel, and cobble spread down from the mountains onto the valley floor, creating a concrete-like lake-bottom hardpan known as caliche -- these days the scourge of the area’s construction companies, swimming-pool installers, and gardeners. Underneath that used to be a major system of artesian aquifers, which was tapped as close to the surface as 40 feet and as deep as 1,000. Before the drastic depletion of the local aquifer in the 1930s, artesian pressure forced this water up to the surface of the valley, creating what came to be known as Big Springs, an oasis of tall grasses, mesquite, and cottonwoods, with several short creeks flowing from it, which converged into the Las Vegas Wash and flowed southeast into the Colorado River (as storm drainage does today into Lake Mead).

People began moving into the Las Vegas Valley as early as 13,000 years ago, at the cold and wet tail end of the Wisconsin Ice Age.

A description of these Paleo-Indians, and the Archaic, Basketmaker, Anasazi, and Paiute cultures that followed, is an answer for another (Question of the) day.