Geologic Events in the History of Great Falls National Park

 

Introduction

The great falls national park is situated in Virginia where beautiful nature and history exist. It is a series of rivers and waterfalls into the Potomac River and runs past, high wall cliffs, and jagged rocks. The rushing whitewater and the jagged rocks of the great falls make it one of the most distinct natural scenes in the area. The great falls of the Potomac, the Mather Gorge, and the National park provide great opportunities for outdoor amusement and also as a research place for nature studies.

 Metagreywacke and mica schist are types of muddy sandstone that have been transfigured by pressure and heat into a hard metamorphic rock Reed, (Sigafoos & Fisher 1980). Before metamorphosis took place, the mica-schist were mudstones and the metagreywacke were beds of muddy sandstone. The noticeable layers are the original layers where the mud and the sand were settled on the bottom of the sea. The rocks have gone through history since they were formed and with time younger sediments assembled above them and they became extremely buried. As the rocks were folded, they were revolved to the edge and were intruded by granite. There was a great change that took place after the emplacement of the granite. 

According to Reed, Sigafoos & Fisher (1980), the earth’s crust ceased moving down and started to rise slowly again, carrying the rocks of the great Falls towards the surface of the earth where they are situated today. Before metamorphosis took place, the schists were mudstones or shales and the metagraywackes were beds of muddy sandstone. The muddy sandstones and the schists were accumulated at the bottom of an ancient ocean. After metamorphosis, the mudstone and the shale were classified into metamorphic gneiss which was formed from chunky deposits of muddy sandstone which contained boulders, quartz, pebbles among other rocks. They were accumulated by huge submarine landslides that gushed through the steep side of the trough.

The overlapping rocks were eroded as the crust rose and the remains were taken westward where it assembled as sedimentary deposits. Due to erosion, the temperatures in the rocks at the great falls went down gradually and the rocks became cracked as they cooled. The straight fractures that cut across the layers of the rocks are called joints (Reed, Sigafoos & Fisher 1980). The most visible joins are almost vertical and form a right angle to the rock layers. The rock is broken into blocks by the joints which makes it easier for the river to quarry away from the firm layers. Across some of the cracks, a different type of molten rock material rose and froze to form the sheets of another rock. Later on, there was moving along the fractures breaking and crushing the rocks along them and displaced the rocks on either side. The sections where such movements have taken place are called faults. The river follows the fault section because the crushed rock along it is easily eroded than the rocks that are not broken. 

Potholes which are circular holes in the ground were caused by pebbles and cobbles that were formed by whirling currents when the water was flowing at this level (Reed, Sigafoos & Fisher 1980). The river smoothened and polished all the rocks in that area at that time. Although there are extreme floods sometimes and reaches this level, the potholes existed much earlier. 

This was even before the formation of the great falls and cutting of gorge. The general shapes of the exposed surface are rounded but all the surfaces of the rock have been made rough by erosion since they were smoothened by the river. Several pebbles that were left by the river can still be seen in the potholes. Large potholes are visible at the present level of the river and other small ones at the top of the cliffs.

There are many pebbles, cobbles, and boulders along the trail that remained behind when the river flowed at this level. They are unique in the area of the Great Falls and the Mather Gorge (Geology 2015). The boulder is the largest and is made up of a dark, heavy igneous rock that is called a diabase. The boulder is extremely large such that it cannot even be moved by the heaviest flood. History is still uncertain of what could have moved it. If it could have been because of the violent floods when the cutting of Mather Gorge happened or perhaps it floated with entangled logs. 

Conclusion

The geologic events in the history of the Great of Great Falls National Park were directed

by the forces within the earth that led to the gathering of residues, compression, and folding. Mather Gorge and the Great Falls exist because of the metamorphic rocks that are resistant to erosion. The rocks formed from sandstones and mudstones and were the earliest to form. The raptures in the rock along the Mather Gorge reveal the past motion along fault lines. The former Potomac River bed is evidenced by circular boulders and polished surfaces and well-formed potholes. Boulders were the largest and were carried from their origin by heavy floods.

 

 

 

           

 

 

 

 

 

References

Geology (2015) Great Falls: National Park Service Retrieved from https:// www.nps .gov/ grfa/

learn/nature/geology.htm

Reed C.J, Sigafoos S.R, &Fisher W.G (1980) The Geologic Story of Great Falls and the Potomac

River Gorge Retrieved from https://pubs.usgs.gov/bul/1471/report.pdf