Trimble Knob, located southwest of

Monterey Monterey (; es, Monterrey; Ohlone: ) is a city located in Monterey County on the southern edge of Monterey Bay on the U.S. state of California's Central Coast. Founded on June 3, 1770, it functioned as the capital of Alta California under both ...

Virginia Virginia, officially the Commonwealth of Virginia, is a state in the Mid-Atlantic and Southeastern regions of the United States, between the Atlantic Coast and the Appalachian Mountains. The geography and climate of the Commonwealth ar ...

in Highland County, is a

conical hill A conical hill (also cone or conical mountain) is a landform with a distinctly conical shape. It is usually isolated or rises above other surrounding foothills, and is often of volcanic origin. Conical hills or mountains occur in different sha ...

composed of

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

, a

volcanic A volcano is a rupture 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 found where tectonic plates a ...

rock, of

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

(early

Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...

) age. It is the eroded remnant of what was an active

volcano A volcano is a rupture 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 found where tectonic plates are ...

diatreme A diatreme, sometimes known as a maar-diatreme volcano, is a volcanic pipe formed by a gaseous explosion. When magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the ...

that last erupted approximately 35 million years ago, making it one of the youngest volcanos on the east coast of

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

Description

Trimble Knob is an isolated conical hill in an otherwise relatively flat valley, surrounded by farmland. The peak of the hill has an elevation of 3123 ft (952m).

U.S. Route 220 U.S. Route 220 (US 220) is a spur route of US 20. It runs in a north–south layout in the eastern United States, unlike its parent route as well as conventionally even-numbered highways which run east-west. US 220 extends for f ...

lies along the southeast flank of the hill. Trimble Knob is the most obvious of many igneous intrusions in the area. The central part of the hill is composed of

with a diameter of approximately . The basalt intrudes through the gently dipping

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

Needmore Formation (

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

iferous shale and calcareous mudstone), and is near the axis of a

syncline In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimpose ...

in the center of the valley.

Age

The basalt at Trimble Knob (and other igneous

dikes Dyke (UK) or dike (US) may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), ''Dikē'', the Greek goddess of moral justice * Dikes ...

in the area) was originally thought to be of

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

age by relative age dating using

cross-cutting relationships Cross-cutting relationships is a principle of geology that states that the geologic feature which cuts another is the younger of the two features. It is a relative dating technique in geology. It was first developed by Danish geological pioneer ...

. In 1993, Southworth and others give a date of 35.0 ± 0.5 Ma for basalt of Trimble Knob In 2012, Bulas and others dated the eruption to 48.86±.37 million years ago. Both dates place the eruption during the

epoch. The basalt intrudes the

Millboro Shale. Mole Hill, located in Rockingham County, is geologically similar to Trimble Knob and thought to be contemporaneous with it, along with other intrusive

igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...

rocks near

Ugly Mountain Ugly Mountain is a summit in Pendleton County, West Virginia, in the United States. With an elevation of , Ugly Mountain is the 507th highest summit in the state of West Virginia. Ugly Mountain is a volcanic neck with a diatreme breccia Brec ...

in Pendleton County,

West Virginia West Virginia is a state in the Appalachian, Mid-Atlantic and Southeastern regions of the United States.The Census Bureau and the Association of American Geographers classify West Virginia as part of the Southern United States while the Bur ...

References

{{coord, 38, 24, 17, N, 79, 35, 16, W, source:enwiki-plaintext-parser, display=title Landforms of Highland County, Virginia Diatremes of the United States Volcanoes of Virginia Eocene volcanoes