A BRIEF GEOLOGIC HISTORY OF ROCKINGHAM COUNTY
By W. Cullen Sherwood
Dept. of Geology and Environmental Studies
James Madison University
FORMING THE MOUNTAINS
Sometime during the 400 million year period when the shallow seas were accumulating sediment which would later become the major rock layers of the county, a curious thing began to happen. The existing Atlantic Ocean apparently began to shrink bringing Europe and Africa closer to North and South America. Finally, approximately 275 million years ago the Atlantic closed and the continents on either side of the ocean collided, ramming northwest Africa into eastern North America. As the continents collided the rocks along the edges were crumpled and broken forming the Appalachian Mountain system. This collision of the continents had a profound effect throughout the eastern United States. The flat sedimentary layers of rocks deposited in the shallow sea were heaved up, folded and broken (see Figure 3) to form mountains much larger than those existing in Rockingham County today. Indeed most of the present mountains of the eastern United States are believed to be the eroded remnants of the great mountains formed during this collision. Figure 3 is a diagramatic sketch of some of the major folds and breaks (faults) which must have existed in our area at the end of the Appalachian mountain building period some 150 million years ago. These mountains were not formed overnight but were produced aver a period of several millions of years and were being affected by rapid erosion as they were formed. The combined action of crumpling the rock layers and rapid erosion must have produced high, rugged mountains in our area similar to the Alps and the Himalayas of today.
|Figure 3. A sketch of the major folds and faults in Rockingham County as they may have appeared some 140 million years ago.
Recent geologic investigations, mostly related to natural gas exploration, have indicated that faulting in the Valley may have been much more intense than was formerly suspected. Apparently in response to the African collision, large slices of rock were broken loose and pushed westward up over other rocks. Some geologists now believe that the Blue Ridge Mountains have been thrust several miles westward up over folded sedimentary rocks similar to those exposed in the valley floor. Drilling planned in this area for natural gas should soon begin to provide some insights into the nature and extent of this faulting and thrusting of the rocks.