Trimble Knob
Encyclopedia
Trimble Knob, located southwest of Monterey
, VA
, in Highland County
, 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
approximately 35 million years ago, making it one of the youngest volcanos on the east coast of North America.
The central part of the hill is composed of basalt
(which in map view has a diameter of approximately 150 m). 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.
in the area) was originally thought to be of Paleozoic
age by relative age dating using cross-cutting relationships. In 1993, Southworth and others give a date of 35.0±0.5 Ma for basalt of Trimble Knob, placing it in the Eocene
. 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
.
Monterey, Virginia
Monterey is a town in Highland County, Virginia, United States. The population was 158 at the 2000 census. It is the county seat of Highland County.-Geography:Monterey is located at ....
, VA
Virginia
The Commonwealth of Virginia , is a U.S. state on the Atlantic Coast of the Southern United States. Virginia is nicknamed the "Old Dominion" and sometimes the "Mother of Presidents" after the eight U.S. presidents born there...
, in Highland County
Highland County, Virginia
As of the census of 2000, there were 2,536 people, 1,131 households, and 764 families residing in the county. The population density was 6 people per square mile . There were 1,822 housing units at an average density of 4 per square mile...
, is a conical hill composed of basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
, a volcanic
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
rock, of Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
(early Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
) age. It is the eroded remnant of what was an active volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
or diatreme
Diatreme
A diatreme is a breccia-filled volcanic pipe that was formed by a gaseous explosion. Diatremes often breach the surface and produce a tuff cone, a filled relatively shallow crater known as a maar, or other volcanic pipes.- Word origin :...
approximately 35 million years ago, making it one of the youngest volcanos on the east coast of North America.
Description
Trimble Knob is an isolated, conical hill in an otherwise relatively flat valley, surrounded by farmland. 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
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
(which in map view has a diameter of approximately 150 m). The basalt intrudes through the gently dipping Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
Needmore Formation
Needmore Formation
The Devonian Needmore Formation or Needmore Shale is a mapped bedrock unit in Pennsylvania, Maryland, Virginia, and West Virginia.-Description:...
(fossiliferous
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
shale and calcareous mudstone), and is near the axis of a syncline
Syncline
In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...
in the center of the valley.
Age
The basalt at Trimble Knob (and other igneous dikesDike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
in the area) was originally thought to be of Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
age by relative age dating using cross-cutting relationships. In 1993, Southworth and others give a date of 35.0±0.5 Ma for basalt of Trimble Knob, placing it in the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
. The basalt intrudes the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
Millboro Shale.
Mole Hill
Mole Hill (Virginia)
Mole Hill, located west of Harrisonburg, VA, in Rockingham County, is a rounded hill composed of basalt, a volcanic rock, of Eocene age...
, located in Rockingham County
Rockingham County, Virginia
As of the census of 2000, there were 67,725 people, 25,355 households, and 18,889 families residing in the county. The population density was 80 people per square mile . There were 27,328 housing units at an average density of 32 per square mile...
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
Pendleton County, West Virginia
As of the census of 2000, there were 8,196 people, 3,350 households, and 2,355 families residing in the county. The population density was 12 people per square mile . There were 5,102 housing units at an average density of 7 per square mile...
, West Virginia
West Virginia
West Virginia is a state in the Appalachian and Southeastern regions of the United States, bordered by Virginia to the southeast, Kentucky to the southwest, Ohio to the northwest, Pennsylvania to the northeast and Maryland to the east...
.