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The land which became Sullivan County was originally purchased from the Iroquois by the Province of Pennsylvania in 1768, as part of the first Treaty of Fort Stanwix.  Prior to the formation of Sullivan County, the area had been included in the New Purchase from the Indians in 1768.  But Connecticut settlers, who had been ousted from the Wyoming Valley, entered and then had to be run off by Pennsylvania agents. Pennsylvania settlers were then, in turn, pushed out by the Indian and Tory attacks of 1778-1780.

Sullivan County was created on March 15, 1847 from part of Lycoming County. According to the official state publication Pennsylvania Local Government, Sullivan County was named for Pennsylvania state senator Charles C. Sullivan, who "took an active part in procuring passage of the bill" establishing the county. Laporte, the county seat, was laid out in 1850 and incorporated as a borough in 1853.  Laporte was named for John LaPorte, Surveyor General of Pennsylvania from 1845-1851.

The Genesee Road from New York opened up the area.  The road allowed settlers to travel from the northern and western areas of what was to become Sullivan County into Bradford County and the Susquehanna River.  , and In 1794 Aristide Dupetit Thouars, a French refugee, was one of the first settlers of the area which later bore a derivation of his name - the town of Dushore.  He was gifted land by the supporters of Marie Antoinette for his support, in what became the town.  He later returned to France to enlist for Napoleon and died on his ship in Egypt on the River Nile going wounded down with his destroyed ship.

In the early 1800's, Lewis Lake (Eagles Mere) was noted for its glass making industry. A woolen industry was productive from 1802 until about 1860 in Forksville. Lumber and related leather tanning industries were most productive throughout the county from 1850 to 1924.  Coal was discovered and mined in several areas of the county. Today, tourism is Sullivan County's major industry, followed by lumbering and agriculture.


According to the U.S. Census Bureau, the county has a total area of 452 square miles (1,170 km2), of which 450 square miles (1,200 km2) is land and 2.6 square miles (6.7 km2) (0.6%) is water.


Elevation ranges from 2593 ft at North Mountain in Davidson Township to 779 ft on Loyalsock Creek at the Lycoming County line. The county is served by Pennsylvania Route 42, Pennsylvania Route 87, Pennsylvania Route 154, Pennsylvania Route 487, and U.S. Route 220. The major rivers in the county are Loyalsock Creek, Little Loyalsock Creek, Muncy Creek, and Fishing Creek. The majority of the land in Sullivan County is forest, but there is some farmland, especially in the northern part of the county. There are numerous river valleys in the southern and western parts of Sullivan County.

Sullivan has a warm-summer humid continental climate and average monthly temperatures in Laporte range from 21.8 °F in January to 67.9 °F in July.


Sullivan County lies predominantly within the Appalachian Plateau physiographic province, which is characterized by gently folded and faulted sedimentary rocks of middle to late Paleozoic age. The southern border of the county is approximately at the Allegheny Front, a geological boundary between the Ridge and Valley province and the plateau. The mountains within Sullivan County are part of the Endless Mountains Region.

The stratigraphic record of sedimentary rocks within Sullivan County spans from the Devonian Lock Haven Formation (exposed only in Lick Creek valley) to the coal-bearing Pennsylvanian Allegheny Formation. Generally, the Catskill Formation underlies most of the lowlands, and sandstones of the Huntley MountainBurgoonMauch Chunk, or Pottsville Formations cap the mountains. No igneous or metamorphic rocks exist within the county, other than possible glacial erratics.

Structurally, the bedrock of Sullivan County is gently folded, with the axes of two major anticlines (including the Wilmot Anticline) and two major synclines (Bernice-Mehoopany Syncline and Noxen Syncline) each trending roughly east–west. There are three mapped faults in the Allegheny Formation between the towns of Murraytown and Ringdale.


Nearly all of Sullivan County was glaciated several times in the past, during the Pleistocene epoch, or "Ice Age." Most of the county is covered by glacial till of Late Wisconsinan age. Late Illinoian Stage deposits may underlie the Late Wisconsinan deposits, and these are exposed in the south central part of the county (roughly western Davidson Township).

The major waterways in Sullivan County are Loyalsock Creek and Muncy Creek. Both flow into the West Branch of the Susquehanna River. Some streams along the eastern border of the county flow into the North Branch of the Susquehanna River. Multiple small tributaries flow into both creeks, some are named but just as many are not. All of Sullivan County is thus within the Chesapeake Bay Watershed.

Several small coal fields exist within Sullivan County. The fields contain either bituminous or semi-anthracite coal, and all occur within Pennsylvanian strata.

Notable geologic features within Sullivan County include some of the following:

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