Although populous, Pennsylvania is relatively small, ranking 33rd among the states in size. It has an area of 119,282 sq km (46,055 sq mi), including 1,269 sq km (490 sq mi) of inland waters and the 1,940 (2000) sq km (749 sq mi) of Lake Erie over which it has jurisdiction. At its maximum, Pennsylvania measures 502 km (312 mi) from east to west and 254 km (158 mi) from north to south. It is bounded on the north by New York and Lake Erie; on the east by New York and New Jersey; on the south by Delaware, Maryland, and West Virginia; and on the west by West Virginia and Ohio.
Pennsylvania may be divided into seven regions based upon differences in landforms. Starting in the southeast, the landform regions are: the Atlantic Coastal Plain, the Piedmont, South Mountain, the Reading Prong, the Ridge and Valley, the Allegheny Plateaus, and the Lake Erie Lowland.
The Atlantic portion of the Coastal Plain in Pennsylvania is part of a vast, low sandy plain that runs along the East Coast of the United States. In Pennsylvania the coastal plain is relatively narrow. Philadelphia is located on this plain. At the western edge of the region the Coastal Plain meets the more resistant rocks of the Piedmont. The Piedmont is an area of foothills that is located between the flat Coastal Plain and the great Appalachian mountain system to the west.
It consists of old crystalline rocks with gently rolling surfaces. Elevations generally range from about 30 to 300 m (about 100 to 1,000 ft). Slopes are moderate, and there are few sharp breaks between hilltops and valley bottoms.
Just west of the Piedmont a narrow tongue of the Blue Ridge Mountains extends into Pennsylvania. This extension is known as South Mountain in both Pennsylvania and Maryland.
Another tongue of mountains lies across the Susquehanna River valley to the northeast of South Mountain. This highland area is known as the Reading Prong.
Geologically, it can be traced through the New Jersey Highlands into the mountainous portions of northern New England. Often South Mountain and the Reading Prong are considered together to be extensions of the Blue Ridge Mountains. Higher points in South Mountain and the Reading Prong rise to about 450 m (about 1,500 ft) above sea level.
The Ridge and Valley region lies to the west of South Mountain and the Reading Prong. It begins with the Great Valley, a fingerlike lowland about 30 km (about 20 mi) wide that extends from the Maryland border in the southwest to the Delaware River in the northeast. The Great Valley is made up of the Lebanon, Lehigh, and Cumberland valleys. Like the Piedmont, the Great Valley is gently rolling. Unlike many valleys, it is not the product of erosion by a single stream but the work of many. Major streams like the Schuylkill and Susquehanna rivers actually cut across the Great Valley, rather than follow it.
Forming the northern and western walls of the Great Valley are the first ridges of the Ridge and Valley region.
The series of long parallel ridges includes Blue, Tuscarora, Jacks, and Bald Eagle mountains. Ridges alternate with long narrow valleys, which are gently rolling lowlands like the Great Valley and the lowlands of the Piedmont. However, the ridges themselves are rough and rocky, and rise more than 300 m (1,000 ft) above the lowlands. Rock in the Ridge and Valley has been steeply folded.
The Allegheny Plateaus is the largest of Pennsylvania’s landform regions. It occupies all of the northern part of the state and much of the west. The plateaus extend into a number of other states, including Ohio and West Virginia. The Allegheny Plateaus region contains the immense Appalachian coalfield, whose bituminous coal deposits extend south into Kentucky, Tennessee, and Alabama. Rocks in this region are flat lying, rather than steeply folded as in the Ridge and Valley. Nevertheless, few parts of the Allegheny Plateaus are level. Most of the region has been deeply etched by streams that branch and rebranch until they resemble the arms of a great tree. Elevations average about 600 m (about 2,000 ft) in the north and 370 m (about 1,200 ft) in southern Pennsylvania.
There are marked regional differences within the Allegheny Plateaus. In the east a small segment is known as the Pocono Plateau or Pocono Mountains. The Poconos are covered with lakes and woodlands. They were once covered by glaciers, as was much of the northern part of the Allegheny Plateaus. Hills have been smoothed and valleys filled, so that differences in elevation are not great.
South of the portions of the Allegheny Plateaus that were glaciated, a great looping arc of highly dissected plateau borders the Ridge and Valley region. This portion of the Allegheny Plateaus contains the Allegheny Mountains, which are bordered on the east by a steep ridge known as the Allegheny Front. The Allegheny Front abruptly separates the Ridge and Valley from the Allegheny Plateaus. The Allegheny Front contains Mount Davis, the highest point in the state at 979 m (3,213 ft) above sea level. The Allegheny Front is cut by an intricate maze of deep valleys. West of the Allegheny Mountains the Allegheny Plateaus region decreases in elevation, and streams have dissected it less deeply. This western portion of the Allegheny Plateaus is sometimes referred to as the Pittsburgh Plateaus.
The seventh landform region found in Pennsylvania is the Lake Erie Lowland, an extension of the Central Lowland. In Pennsylvania the region is only a slim strip of land from 5 to 8 km (3 to 4 mi) wide along the southern shore of Lake Erie. The ascent toward the south from Lake Erie to the Allegheny Plateaus is achieved by a series of glacial terraces, or steps, in the lowland plain. The terraces were formed by wave action when Lake Erie was larger long ago. Encarta "Pennsylvania" © Emmanuel BUCHOT, Encarta, Wikipedia
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