Trimble Knob

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

Trimble Knob
Photo of Trimble Knob, taken December 2011
Highest point
Coordinates38°24′17″N 79°35′16″W 
Geography
LocationSouthwest of Monterey, Virginia
Geology
Age of rock35.0 ± 0.5 Ma
Mountain typeEroded volcano or diatreme
Last eruption35 Ma

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 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 with a diameter of approximately 150 metres (160 yd). The basalt intrudes through the gently dipping Devonian Needmore Formation (fossiliferous shale and calcareous mudstone), and is near the axis of a syncline in the center of the valley.[3]

Age

The basalt at Trimble Knob (and other igneous dikes in the area) was originally thought to be of Paleozoic age by relative age dating using 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 dated the eruption to 48.86±.37 million years ago.[5] Both dates place the eruption during the Eocene epoch. The basalt intrudes the Devonian 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 rocks near Ugly Mountain in Pendleton County, West Virginia.[1]

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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". Circular 1264. United States Geological Survey.
  2. elevationmap.net. "Trimble Knob United States on the Elevation Map. Topographic Map of Trimble Knob United States". elevationmap.net. Retrieved 2018-06-11.
  3. 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.
  4. 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.
  5. Guzman and Johnson. "Evidence for a Two-Stage Eruption at Trimble Knob, an Eocene Volcanic Plug in Highland County, VA" (PDF). Retrieved 11 June 2018.

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