Trimble Knob

On Friday afternoon and Saturday I continued my trips into the mountains to learn more about Appalachian geology and paleontology. As we drove toward Monterey, Virginia on US 220, we passed by Trimble Knob, a large conical hill that sticks up prominently above the surrounding landscape.

Trimble Knob is apparently privately owned, with a fence around it, so we couldn’t go up. However, there are roads all the way around it, so we were able to get good pictures, including zooming in on some of the exposed rocks:

Trimble Knob is made primarily of basaltic igneous rocks, which are not especially common in the Appalachians. There are actually a series of comparable structures, as well as igneous dikes, in Highland and Rockingham Counties in Virginia and Pendleton County in West Virginia. Trimble Knob (which is the largest of these structures) seems to be a diatreme. Diatremes are formed when magmas rise up to near the surface and interact with groundwater. The groundwater flashes to steam and explodes, opening a gradually widening crater that fills with a mixture of the cooled magma and fractured pre-existing (country) rock (in this case, Devonian Needmore Shale). The resulting structure is more resistant to erosion than the shale, and ends up as a high-relief feature.

The vast majority of surface igneous events are related to motion at tectonic plate boundaries, either rifting during the formation of an ocean basin or subduction when an ocean basin is destroyed. For decades after the discovery of Trimble Knob and related features, they were assumed to be Triassic in age, related to the opening of the Atlantic Ocean.

This all changes in 1969, when Fullagar and Bottino used K-Ar and Rb-Sr dating to determine that these rocks were Eocene in age, more than 200 million years younger than previously supposed. The structures they dated were only about 47 million years old; Southworth et al. (1993) later reported a K-Ar date of 35 million years (± 0.5 Ma) for Trimble Knob, making it the youngest known igneous structure in the eastern United States.

These dates are rather remarkable, because there was neither a rifting event nor a subduction zone of the east coast of North America at that time. There have been a number of hypotheses proposed for the origin of these rocks (see Tso et al. 2004 for a review). It seems the most accepted proposal was that made by Southworth et al. (1993). They suggest that during the Eocene the direction of motion of the North American plate changed, relieving compressional stress on the crust. This opened pre-existing fractures enough to allow some magma to rise rapidly to the surface, causing a small amount of igneous activity that lasted about 12 million years. The same change in plate motion is thought to possibly be responsible for the formation of the Bermuda Rise at the same time.

We also actually found some fossils on our trip; more on those in a few days.


Fullagar, P. D. and M. L. Bottino, 1969. Tertiary felsite intrusions and associated phenomena near the 38th parallel fracture zone in Virginia and West Virginia. Geological Society of America Bulletin, 82:501-508. (Abstract)
Tso, J. L., R. R. McDowell, K. L. Avary, D. L. Matchen, and G. P. Wilkes, 2004. Middle Eocene igneous rocks in the Valley and Ridge of Virginia and West Virginia. U. S. Geological Survey Circular 1264.
Southworth, C. S., K. J. Gray, and J. F. Sutter, 1993. Middle Eocene intrusive igneous rocks of the Central Appalachian Valley and and Ridge Province — setting, chemistry, and implications for crustal structure. U. S. Geological Survey Bulletin 1839, J1-J24.
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6 Responses to Trimble Knob

  1. Keith D says:

    When I was in college, I remember hearing that the hot springs in that region were the resilt of Cenozoic igneous activity. I don’t remember anything about volcanism – I was thinking magma not lava. Then again I wanted to be a paleontologist, so I may have missed a detail or two when the professor talked about igneous stuff….

  2. Alton Dooley says:

    I hadn’t heard of the link to the local hot springs, but it does make sense. They’re pretty close; we drove through Hot Springs and Warm Springs on the way to Monterey.

  3. Keith D says:

    I have been thinking about this all week. Isn’t the Chesapeake Bay crater dated at 35 million years? So maybe there was a reactivation caused by changes in plate motion in the 45 million year range, then re-reactivation caused by bolide impact around 35 millions years.

  4. Alton Dooley says:

    Maybe; the Bay impact does date to about 35 Ma. However, Trimble Knob is just one of a swarm of igneous structures in the area. Mole Hill in Harrisonburg I think is the oldest, at 47 Ma, but there are bunches of knobs and dikes all through the area, and as far as I know the dates don’t have a bimodal age distribution; they seem to be pretty evenly distributed between 47 and 35 Ma. That would suggest a single originating cause for all the features.

    As an aside, on Tuesday night I gave a lecture at Radford University and had a chance to speak briefly with Jonathan Tso, who has done some of the work on Trimble Knob. These diatremes are marked by a breccia composed of basalt and country rock, and Jonathan told me that the breccia around Trimble Knob not only contains identifiable fragments of Devonian sediments, but that these fragments contain recognizable fossils.

  5. Charles says:

    As a Highland County native, i can tell you that accross Monterey Mountain, in the Blue Grass, Valley; there is alot of volcanic rock, particularly on the old George Swecker Farm. It has rather large holes all in it and is very light in relation to its size. Could it have come from trimble’s knob, or some other volcano / vent etc.?

  6. altondooley says:

    I don’t know where that specific locality is, but if they are igneous rocks they very likely have the same origin as Trimble Knob. Those features are scattered all across that part of Virginia, and across the border into West Virginia.

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