# Trimble Knob

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**Trimble Knob**, located southwest of [Monterey](/source/Monterey,_Virginia) in [Highland County](/source/Highland_County,_Virginia), [Virginia](/source/Virginia), is a [conical hill](/source/Conical_hill) composed of [basalt](/source/Basalt), a [volcanic](/source/Volcano) rock, of [Eocene](/source/Eocene) (early [Tertiary](/source/Tertiary_period)) age. It is the eroded remnant of what was an active [volcano](/source/Volcano) or [diatreme](/source/Diatreme) that last erupted approximately 35 million years ago, making it one of the youngest volcanos on the east coast of [North America](/source/North_America).[1]

## 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).[2] [U.S. Route 220](/source/U.S._Route_220_in_Virginia) 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 [basalt](/source/Basalt) with a diameter of approximately 150 metres (160 yd). The basalt [intrudes](/source/Intrusive_rock) through the gently dipping [Devonian](/source/Devonian) [Needmore Formation](/source/Needmore_Formation) ([fossiliferous](/source/Fossil) shale and calcareous mudstone), and is near the axis of a [syncline](/source/Syncline) in the center of the valley.[3]

## Age

The basalt at Trimble Knob (and other igneous rock [dikes](/source/Dike_(geology)) in the area) were originally thought to be of [Paleozoic](/source/Paleozoic) age by relative age dating using [cross-cutting relationships](/source/Principle_of_cross-cutting_relationships).[1] In 1993, Southworth and others give a date of 35.0 ± 0.5 Ma for basalt of Trimble Knob[4] In 2012, Bulas and others studied the other igneous rocks and sampled them to obtain the mineral separates and perform age dating. The amphibole minerals composing the volcanic rock Trachydacite were dated utilizing the Ar-Ar age dating method and found them to be 48.86±.37 million years of age. The age of the volcanic eruption is not explained by the age of the formation of amphibole minerals which formed at a time before the volcanic eruption occurred. There may have been a bi-model or two-stage eruption mechanism responsible for producing the different varieties of mafic and felsic volcanic rock that have been discovered and associated with this volcanism.[5] Both age dates place the earliest age of the eruption to be during the [Eocene](/source/Eocene) epoch and further research is needed to better understand the timing and eruption mechanisms responsible for these rock types.

[Mole Hill](/source/Mole_Hill_(Virginia)), located in [Rockingham County](/source/Rockingham_County,_Virginia), is geologically similar to Trimble Knob and thought to be contemporaneous with it, along with other intrusive [igneous](/source/Igneous) rocks near [Ugly Mountain](/source/Ugly_Mountain) in [Pendleton County](/source/Pendleton_County,_West_Virginia), [West Virginia](/source/West_Virginia).[1]

## References

1. Jonathan L. Tso; Ronald R. McDowell; Katharine Lee Avary; David L. Matchen; & Gerald P. Wilkes (2004). ["Middle Eocene Igneous Rocks in the Valley and Ridge of Virginia and West Virginia"](https://pubs.usgs.gov/circ/2004/1264/html/trip4/index.html). *Circular 1264*. [United States Geological Survey](/source/United_States_Geological_Survey)

1. elevationmap.net. ["Trimble Knob United States on the Elevation Map. Topographic Map of Trimble Knob United States."](https://elevationmap.net/trimble-knob-united-states). *elevationmap.net*. Retrieved 2018-06-11.

1. Rader, E.K. and Wilkes, G.P., 2001, Geologic map of the Virginia portion of the Staunton 30 X 60 minute quadrangle: Virginia Division of Mineral Resources, Publication 163, scale 1:100000.

1. Southworth, C.S., Gray, K.J., and Sutter, J.F., 1993, Middle Eocene intrusive igneous rocks of the central Appalachian Valley and Ridge province; Setting, chemistry, and implications for crustal structure: U.S. Geological Survey Bulletin 1839, p. J1–J24. [\[1\]](https://pubs.usgs.gov/bul/1839i/report.pdf)

1. Guzman and Johnson. ["Evidence for a Two-Stage Eruption at Trimble Knob, an Eocene Volcanic Plug in Highland County, VA"](https://gsa.confex.com/gsa/2012AM/webprogram/Handout/Paper206574/Trimble%20Knob%20GSA%20%28print%201%20HQ%29.pdf). Retrieved 11 June 2018.

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Adapted from the Wikipedia article [Trimble Knob](https://en.wikipedia.org/wiki/Trimble_Knob) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Trimble_Knob?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.