VALLEY FORGE NATIONAL PARK
When viewing this site and reading through the geologic history, it is important to remember that the Earth's geologic history covers a time span of about 4.6 billion years! The oldest rocks presently exposed in Valley Forge National Park are approximately 525 million years old, which means that only a small portion of the Earth's entire history is recorded within the park. However, through techniques of geologic interpretation, scientists have constructed a timeline of geologic events that affected the Valley Forge area during the Paleozoic, Mesozoic, and Cenozoic eras.
Figure borrowed from http://www.tyrrellmuseum.com/tour/timescal.html
The geologic time scale above separates the geologic history of the Earth into major Eras and Periods. The ages of the major divisions are shown in millions of years before the present. Where does the geologic history of Valley Forge National Park fall within the bigger picture? Major events in the park's geologic history took place during the Paleozoic, Mesozoic and Cenozoic Eras. Read below to discover the specifics of these event. The oldest geologic events are discussed at the top of the page and more recent history is discussed at the bottom.
About 525 million years ago during the Cambrian period, the coastline of the ancient North American continent was flooded by rising seas. At this time, sand accumulated along beaches, while muds and limy sediments were deposited offshore. Over time, the continued rise in sea level produced a layered stack of sedimentary rocks having sandstone at the base and carbonate rocks at the top. In Valley Forge, the sediments deposited during this time were primarily sandstones, limestones, and dolostones. Marine life included trilobites, brachiopods, bryozoans, snails, clams, and sponges. The Cambrian Period marks the first time that multicellular organisms are known to have existed, but none of these organisms are noted in the rocks presently exposed at the surface within Valley Forge.
|Cambrian Chickies Quartzite. The Chickies formation underlies Mount Misery and Mount Joy, two topographic high points within the park. The unit consists largely of sand-sized quartz grains which probably were deposited in a beach environment. The original sandstone has been metamorphosed to quartzite under the influence of heat and pressure.|
|Cambrian Ledger Dolostone. The Ledger dolostone underlies the area east and south of Mount Joy. Dolostone consists predominantly of the mineral dolomite (calcium and magnesium carbonate). Some carbonates form as chemical precipitates directly from sea water. Others are formed by marine organisms, such as coral, which extract calcium and carbonate from seawater to construct their shells.|
The geologic setting began to change about 480 million years ago during the Ordovician period from a period of sediment accumulation to one of sediment erosion and rock deformation. For the next 200 million years, the Valley Forge area experienced intervals of mountain building activitiy that subjected rocks to deforming forces accompanied by increases in temperature and pressure. The most intense event to affect the region occurred near the beginning of this period when a volcanic island chain, much like the Japanese archipelago, collided with the North American continent. The collision caused originally horizontal layers of sedimentary rocks to be compressed into folds and to be thrust inland along large faults.
Figure borrowed from http://xray.geol.uni-erlangen.de/html/teaching/plate/pla_tec.html
At the end of the Paleozoic Era, all of the earths continents were assembled into a single landmass called Pangaea. Africa had collided with North America, producing the Appalachian Mountains in central Pennsylvania. Once the mountain-building activity ceased, natural weathering processes began wearing down the mountains.
About 200 million years ago, Pangea began to split apart. This rifting event marked the birth of the Atlantic Ocean. The separation of Africa from North America was accompanied by a stretching of the crust over a broad area. In response to the stretching, blocks of the Earths crust dropped down along faults, forming long, narrow valleys, or basins. The park is located near the southern edge of one of these, the Newark basin. The basin is filled with sedimentary rocks that are stained red as a result of the formation of rust when iron in the sediments combined with oxygen. The basin also contains rocks that were once molten but have since cooled and solidified called igneous rocks. In places, the basin-filling rocks are over four kilometers thick.
|Triassic Stockton Formation. The red conglomerates, sandstones, siltstones, and shales of the Stockton Formation underlie the northern area of the park. These were deposited around 200 million years ago in a river setting. Rainwaters carried pebble, sand, and silt-sized particles from highlands into a wide river channel and its adjacent floodplain. Particles of clay and silt settled in lakes scattered across lowland areas. During dry spells, the lakes dried up, leaving exposed mudflats.|
|Red soils develop in areas underlain by the red sedimentary rocks of the Stockton Formation.|
For about the last 150 million years or so, the eastern half of North America has been geologically quiet. Under such conditions, weathering and erosion acts as the dominant agent shaping the land surface. The present topography of the park is the result of these processes.
|Professor Crawford discusses stream dynamics along the banks of Valley Creek. Here, one can observe the unconsolidated stream deposits which make up the youngest geologic formation within the park.|
|Valley Creek overflows its banks during heavy rain events. During the rainfall that accompanied Hurricane Floyd (September, 1999), the sediment-laden water deposited sands and mud over the floodplain flattening the tall river grasses along the shores.|