Currently, the geological time scale is divided into three kinds of geological units: rock, time rock and time units. The rock units describe the physical (lithologic) characteristics of the rocks while the time units refer only to the passage of time. In line with this, the Cenozoic era refers to the time between 65 million years ago and today. It refers to the rocks that were deposited during the last 65 million years, and these rocks may have been altered during breaks in this record. These alterations could be caused by erosion, nondeposition or even nonrecognition and are largely responsible for the formation of the geological landscape of Virginia, specifically the western side. During this Cenozoic era, the rivers spread the sand and gravel over the tertiary sediments, and there were a lot of changes that the Virginia region experienced. Hence, Virginia landscape can largely be attributed to the changes in the Cenozoic era, and its history is more evident in the topography than the sedimentary rocks.
The Local Geology of Virginia
The geologic division of Virginia is such that it coincides with the physiographic divisions. That is, each one of the provinces in the region is differentiated by various groups of rocks which are sedimentary, igneous and metamorphic rocks. On the other hand, the boundaries tend to be delimited by the character and the structure of its rocks.
The geology of the coastal plain is quite simple. They are in such a way that the beds of the sedimentary rock dip towards the sea and they are exposed in successively wider belts from the coast towards the fall line. There are clays, sandstones, igneous and metamorphic come in various ranges from the loose to the well-indurated materials. There are superficial mantels of sand and gravel in the coastal streams and the Dismal Swamp that it located in the southwest of Norfolk is characterized by peat. The basement rocks of the coastal plain are crystalline, like those in Piedmont Province. They are igneous and metamorphic and they about in granites, gneisses, schists and greenstones. In the southwest, there are much younger red sandstone and shale with the most extensive of these formations being in the northwestern part of the Piedmont Province and the in the Richmond basin.
The valley and the ridge province are predominantly covered in sedimentary rocks, and the Valley of Virginia is covered in limestone (Paleozoic). However, there are also broad belts of shale. In the valley and the ridge sections towards the west, there are beds of sandstone that support the ridges and their crests. Cropping of limestone and shale exist along the slopes with the majority of the intermontane valleys being on shale and sandstone while others being predominantly limestone.
Further, the sedimentary strata in the Appalachian Valley exhibit a series of great anticline and synclines. The folds are designed in a manner that they are overturned northwestwards, and they trend southwest towards the ridges (Stanley 6). Most of them have been broken into fractures or faults, such that large blocks or long thick horizontal slices of the earth’s crust are shoved towards the northern side. From the faults, there have been marked repetition of outcrop in the various limestone and other formations that further make the structure and the diversity of the topography much more complex. From the sequence of their formation, the bedrocks in Virginia are a representation of the periods in the four more recent areas of the geologic time frame (Stanley 7). The pre-Cambrian rocks of igneous and the metamorphic origin are found in the surface of the Piedmont region in the Blue Ridge. The Paleozoic rocks are found in the west of the Blue Ridge. The Mesozoic rocks that are found in Virginia crop out in Piedmont along the Western Ridge of the coastal plain. Most parts of the coastal region are covered by Tertiary sands, clays, and gravels.
The region is also defined by fossil remains of the marine invertebrates, and they are confined to the coal beds and shales that are associated with such beds. They include various ferns, conifers and rushes that have been extinct (Sullivan 57). There have also been scanty features of some vertebrates of the Cenozoic age that saw the rise and dominance of the mammals. These fossils remain of the composition of the majority of the rocks in the region thereby defining the nature of the rocks in the region.
During the subsequent Mesozoic and Cenozoic eras, there was the formation of a plain that was uplifted in the Western Virginia. In the early part of the Tertiary period of the Cenozoic period, Virginia was eroded to a nearly featureless peneplain. The region was once again lifted through a slight warping. There were erosive forces that etched into the relief and the entered the more resistant rocks in the region and formed weaker valleys in the weaker strata. It explains the reason as to why the region was uplifted with slight warping forming valleys with weaker strata. As a result of these processes, there has been formation numerous ridges in Western Virginia are capped with sandstone and conglomerate. The Upper Carboniferous rocks being that they are weaker have undergone erosion more than the underlying rock (Parker and Gary 80). Besides, the western part of the state has been reduced to a low-lying plain that is close to the sea level. It has led to a situation whereby there are streams that meander over the land, resulting into wide flood plains. Towards the end of the Tertiary period, there was a second uplift that occurred in the region and streams that gave the region the sculpture that it has currently.
The Pleistocene continental ice sheets modified drainage in the area. Prior to the ice age, Lake Erie was not in existence and the streams of Western Virginia did flow across the present Ohio River to the Erie basin and finally to St. Lawrence River (Virginia 67). From the recent geological changes that have taken place in the region, the Kanhwa drainage system have been such that it has been robbed of the rivers that flow towards the west. Due to the uplift that initially took place and in part to the resistant Carboniferous sandstones, the river entrenched itself to a gorge to a plain below the peneplain level.
The formation of rocks can be attributed to several natural processes most of which have been modified by natural phenomenon. It has resulted in the kind of topography that Virginia has and has been the main driver of the economic and the social conditions of the state. The rocks have also been formed as a result of the various fossil depositions that have taken place over the years thereby being the main composition in some of the rocks in various regions.
Stanley, Steven M. Earth System History: 3rd Edition. New York: W.H. Freeman & Co Ltd, 2008. Print.
Virginia: The Old Dominion. New York: AV2 by Weigl, 2011. Print.
Sullivan, Ken. The West Virginia Encyclopedia. Charleston, W. Va: West Virginia Humanities Council, 2006. Print.
Parker, Gary, and Mary M. Parker. The Fossil Book. Green Forest, AR: Master Books, 2005. Print.