A BRIEF GEOLOGIC HISTORY OF ROCKINGHAM COUNTY
By W. Cullen Sherwood
Dept. of Geology and Environmental Studies
James Madison University
MOLE HILL AND OTHER MELTED ROCKS
Formation of the mountains left the originally flat layers of Rockingham extensively folded and broken. Avenues of weakness were now available whereby any melted rock material formed at depth in the earth could find its way up to or near the surface. We have good evidence in Rockingham County that several masses of molten (igneous) material did find their way upward into the think overlying sedimentary layers and cooled forming hard rocks. A number of these formerly molten rocks are exposed at the surface in Rockingham County today.
Most igneous rocks found in Rockingham formed as vertical thin tablet-shaped bodies known as dikes. Most are only a few feet thick, and extend downward like a curtain into the earth. These dikes can often be found in the field by looking for brown, rounded rocks on the surface. Two dikes well known to local geologists are located as follows: one just east and almost parallel to Rt. 910 north of Rt. 765 (this dike crosses Rt. 910 just north of Green Mount Church), the other is exposed in the field just east of the Mundy Quarry and west of Rt. 602 in east Rockingham. So far as is known all of the thin dikes or igneous intrusions in the county are composed of a nearly black igneous rock.
Probably the most interesting igneous intrusion in Rockingham County is that exposed in Mole Hill located approximately four miles southwest of Harrisonburg near Dale Enterprise (see figure 4).
|Figure 4. A cut away view of Mole Hill. This magnified view shows the olivine crystals and fragments of sandstone carried upward by the melted rock. Above in dashed lines is a volcano which may have existed millions of years ago but has now been eroded away.
The rock here is also dark colored with large crystals of the green, glassy mineral known as olivine included in it. Also included in the rock are pieces of sandstone which could only have been picked up thousands of feet down and brought to the surface as the lava flowed upward. Evidence is strong that Mole Hill is the conduit or pipe which fed molten igneous rock to a volcano which has now been eroded away. Recent work by Wampler and Dooley (1975) shows an age of approximately 47 million years for the Mole Hill rocks. This date is considerably younger than the 180 to 230 million year dates formerly attributed to the igneous rocks of the Valley. So at this point we can say that Mole Hill may well have been one of the last volcanoes which existed in what is now Virginia.