Leonard Harrison State Park - Geology and Climate

Geology and Climate

See also: Climate of Pennsylvania

Although the rock formations exposed in Leonard Harrison State Park and the Pine Creek Gorge are at least 300 million years old, the gorge itself formed only about 20,000 years ago, in the last ice age. Pine Creek had flowed northeasterly until then, but was dammed by rocks, soil, ice, and other debris deposited by the receding Laurentide Continental Glacier. The dammed creek formed a lake near the present village of Ansonia, and the lake's glacial meltwater overflowed the debris dam, which caused a reversal of the flow of Pine Creek. The creek flooded to the south and quickly carved a deep channel on its way to the West Branch Susquehanna River.

The park is at an elevation of 1,821 feet (555 m) on the Allegheny Plateau, which formed in the Alleghenian orogeny some 300 million years ago, when Gondwana (specifically what became Africa) and what became North America collided, forming Pangaea. While the gorge and its surroundings appear mountainous, these are not true mountains: instead years of erosion have made this a dissected plateau, causing the "mountainous" terrain seen today. The hardest of the ancient rocks are on top of the ridges, while the softer rocks eroded away forming the valleys.

The land on which Leonard Harrison State Park sits has undergone tremendous change over the last 400 million years. It was once part of the coastline of a shallow sea that covered a great portion of what is now North America. The high mountains to the east of the sea gradually eroded, causing a buildup of sediment made up primarily of clay, sand and gravel. Tremendous pressure on the sediment caused the formation of the rocks that are found today in the Pine Creek drainage basin: sandstone, shale, conglomerates, limestone, and coal.

Five major rock formations are present in Leonard Harrison State Park, from the Devonian and Carboniferous periods. The youngest of these, which forms the highest points in the park and along the gorge, is the early Pennsylvanian Pottsville Formation, a gray conglomerate that may contain sandstone, siltstone, and shale, as well as anthracite coal. Low-sulfur coal was once mined at three locations within the Pine Creek watershed. Below this is the late Mississippian Mauch Chunk Formation, which is formed with grayish-red shale, siltstone, sandstone, and conglomerate. Millstones were once carved from the exposed sections of this conglomerate. Together the Pottsville and Mauch Chunk formations are some 300 feet (91 m) thick.

Next below these is the late Devonian and early Mississippian Huntley Mountain Formation, which is made of grayish-red shale and olive-gray sandstone. This is relatively hard rock and forms many of the ridges. Below this is the red shale and siltstone of the Catskill Formation, about 760 feet (230 m) thick and some 375 million years old. This layer is relatively soft and easily eroded, which helped to form the Pine Creek Gorge. Cliffs formed by the Huntley Mountain and Catskill formations are visible north of the park at Barbour Rock. The lowest and oldest layer is the Lock Haven Formation, which is gray to green-brown siltstone and shale over 400 million years old. It forms the base of the gorge, contains marine fossils, and is up to 600 feet (180 m) thick.

The dominant soil in Leonard Harrison State Park is somewhat excessively drained Oquaga channery loam, which is often associated with well drained Lordstown channery loam. Much of the campground near the eastern boundary is supported by Morris gravelly silt loam, which is somewhat poorly drained due to a subsoil fragipan. The Oquaga tends to be very strongly acidic (pH 4.8), Morris is strongly acidic (pH 5.3) and Lordstown is moderately acidic (pH 5.5). All of these soils belong to the Inceptisol soil order.

The Allegheny Plateau has a continental climate, with occasional severe low temperatures in winter and average daily temperature ranges of 20 °F (11 °C) in winter and 26 °F (14 °C) in summer. The mean annual precipitation for the Pine Creek watershed is 36 to 42 inches (914 to 1,070 mm). The highest recorded temperature at the park was 104 °F (40 °C) in 1936, and the record low was −30 °F (−34 °C) in 1934. On average, July is the hottest month at Leonard Harrison, January is the coldest, and June the wettest.

Climate data for Leonard Harrison State Park
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Average high °F (°C) 30
Average low °F (°C) 13
Precipitation inches (mm) 1.88
Source: The Weather Channel

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