Table Mountain is a narrow, -long, sinuous, flat-topped ridge separated by erosional saddles into a series of

mesa A mesa is an isolated, flat-topped elevation, ridge, or hill, bounded from all sides by steep escarpments and standing distinctly above a surrounding plain. Mesas consist of flat-lying soft sedimentary rocks, such as shales, capped by a ...

s that extend from Lake Tulloch to just west of

Columbia, California Columbia is a census-designated place (CDP) located in the Sierra Nevada foothills in Tuolumne County, California, Tuolumne County, California, United States. It was founded as a boomtown in 1850 when gold was found during the California Gold R ...

Tuolumne County, California Tuolumne County (), officially the County of Tuolumne, is a County (United States), county located in the U.S. state of California. As of the 2020 United States census, 2020 census, the population was 55,620. The county seat and only incorpora ...

. It is just over in elevation at its southern end and just over in elevation at its northern end. Its flat top is part of a stack of multiple -long lava flows that have been eroded to form a series of mesas that extend from Knights Ferry to

Sonora, California Sonora is the only incorporated city in Tuolumne County, California, United States, of which it is also the county seat. Founded during the California Gold Rush by Mexican miners from Sonora (after which the city is named), the city population ...

. Its crest varies in width from a narrow ridge to over wide. It parallels the adjacent

Stanislaus River The Stanislaus River is a tributary of the San Joaquin River in north-central California in the United States. The main stem of the river is long, and measured to its furthest headwaters it is about long. Originating as three forks in the h ...

Geology

The

lava flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

s that form the sinuous flat-topped mesas of Table Mountain in Tuolumne County belong to a geologic formation known as the Table Mountain Latite. Within the region of Table Mountain, the Table Mountain Latite is mapped as a part of the Mehrten Formation. Further north, it is considered part of a sequence of volcanic strata known as the ''Stanislaus Group''. The Table Mountain Latite consists of high-

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

trachyandesite Trachyandesite is an extrusive igneous rock with a composition between trachyte and andesite. It has little or no free quartz, but is dominated by sodic plagioclase and alkali feldspar. It is formed from the cooling of lava enriched in alkal ...

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

s. During the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Ma, these lavas were likely erupted from the Little Walker Volcanic Center near

Sonora Pass Sonora Pass (el. 9,624 ft. / 2,933 m.) is a mountain pass in the Sierra Nevada in California. It is the second-highest pass with a road in California and in the Sierra Nevada. It is lower than Tioga Pass to the south. State Route 108 trav ...

. They flowed more than down a palaeo-Stanislaus River channel, known as the ''Cataract Channel'', and past Knights Ferry in the

Sierra Nevada The Sierra Nevada ( ) is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primari ...

foothill Foothills or piedmont are geographically defined as gradual increases in elevation at the base of a mountain range, higher hill range or an upland area. They are a transition zone between plains and low relief hills and the adjacent topograp ...

s. These lava flows filled the Cataract Channel and overflowed into parts of its

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river. Floodplains stretch from the banks of a river channel to the base of the enclosing valley, and experience flooding during periods of high Discharge (hydrolog ...

. Before the creation of

New Melones Lake New Melones Lake is a reservoir on the Stanislaus River in the central Sierra Nevada foothills, within Calaveras County and Tuolumne County, California. The New Melones Dam and reservoir are a water collection and transfer unit of the United ...

and Lake Tulloch, the modern Stanislaus River occupied a deep gorge west of Table Mountain. Beneath the Table Mountain Latite and forming the bulk of Table Mountain are

andesitic Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...

mudflow A mudflow, also known as mudslide or mud flow, is a form of mass wasting involving fast-moving flow of debris and dirt that has become liquified by the addition of water. Such flows can move at speeds ranging from 3 meters/minute to 5 meters/se ...

s and conglomerates of the Mehrten Formation. They consist predominantly of

debris flow Debris flows are geological phenomena in which water-laden masses of soil and fragmented Rock (geology), rock flow down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. ...

deposits (interpreted as

volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

lahar A lahar (, from ) is a violent type of mudflow or debris flow composed of a slurry of Pyroclastic rock, pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a valley, river valley. Lahars are o ...

s), and

fluvial A river is a natural stream of fresh water that flows on land or inside caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of its course if it ru ...

conglomerates, lithic

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

s, and

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

s deposited by rivers which flowed westward across the Table Mountain region. Underlying the Mehrten Formation is the much thinner Valley Springs Formation. It consists of sandstone, siltstone and conglomerate interbedded with altered volcanic ashflow

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...

s. The Valley Springs Formation represents sediments that were deposited by westward flowing river systems and intermittent channelized volcanic ashflows (

pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...

s) within the same drainage system. The sediments of the Mehrten and Valley Springs formations once formed a thick blanket of volcanoclastic

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

s which almost completely buried the pre-

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

topography and river valleys of this region. At the end of their deposition, the only pre-

Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

basement rocks In geology, basement and crystalline basement are crystalline rocks lying above the mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment. The baseme ...

exposed in this region were ridges of resistant greenstone (a type of

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

metavolcanic rock Metavolcanic rock is volcanic rock that shows signs of having experienced metamorphism. In other words, the rock was originally produced by a volcano, either as lava or tephra. The rock was then subjected to high pressure, high temperature or both ...

) and a few high summits of the Sierra Nevada's middle slopes. Further north and closer to their source areas, the Mehrten and Valley Springs formations consist of volcanic and volcanoclastic deposits that have been divided into several stratigraphic units, e.g. the Relief Peak Formation, Stanislaus Group, etc., based on their age and source volcanic center. Lying buried beneath the volcanoclastic sediments of the Mehrten and Valley Springs formations within the Table Mountain region are paleovalleys cut into pre-Cenozoic bedrock. These paleovalleys are filled with

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

, prevolcanic,

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

-rich,

gravel Gravel () is a loose aggregation of rock fragments. Gravel occurs naturally on Earth as a result of sedimentation, sedimentary and erosion, erosive geological processes; it is also produced in large quantities commercially as crushed stone. Gr ...

ly,

gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

-bearing stream deposits. Since the early days of gold mining in the Sierra Nevada, these gravelly sediments have been known as the ''Auriferous gravels.'' Although this name is inappropriate because gold occurs in youngers gravels of younger age and the rules of naming stratigraphic rock units, no one has yet suggested a better name for the Auriferous gravels. The Auriferous gravels are distinct from the younger

deposits in that they lack Cenozoic volcanic material. By 1911, Lindgren had largely reconstructed the courses of these ancient river valleys and systems cut into basement rocks and filled with the gold-bearing Auriferous gravels. Where exposed in modern valleys these channels have been extensively mined for gold by

hydraulic mining Hydraulic mining is a form of mining that uses high-pressure jets of water to dislodge rock material or move sediment.Paul W. Thrush, ''A Dictionary of Mining, Mineral, and Related Terms'', US Bureau of Mines, 1968, p.560. In the placer mining of ...

and underground they have been thoroughly explored by

drift mining Drift mining is either the mining of an ore deposit by underground methods, or the working of coal seams accessed by adits driven into the surface outcrop of the coal bed. A drift mine is an underground mine in which the entry or access is abo ...

. Although exceedingly rich in placer gold, they have been completely exhausted as a source of commercial placer gold in Table Mountain area. Since the lava flows of the Table Mountain Latite filled the Cataract Channel circa 10.4 Ma, erosion has stripped the Mehrten and Valley Springs formations from the Table Mountain region where they were not covered and protected by the lava flows of the Table Mountain Latite. As a result, the prevolcanic unconformity lying at the base of the Mehrten and Valley Springs formations has been exhumed and is now exposed as the modern rolling topography adjacent to Table Mountain.

Ecology

A variety of flora and fauna are found on Table Mountain. In the spring, many

wildflowers A wildflower (or wild flower) is a flower that grows in the wild, rather than being intentionally seeded or planted. The term implies that the plant is neither a hybrid nor a selected cultivar that is any different from the native plant, eve ...

can be found atop the mountain including several species of lupine and the yellow mariposa lily, ''Calochortus luteus''. Vernal pools form on the flat top after heavy winter and spring rains, providing habitat for many uncommon plant and animal species. The harsh conditions found on top of Table Mountain generally prevent invasive plants from surviving, resulting in a landscape which is composed primarily of native plants. White-throated swifts nest on the cliff faces that flank the mountain.

References

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Geology

Ecology

References

See also