Colerain Township was formed in the year 1767 within Cumberland County. It was one of the original five townships formed out of the frontier of Cumberland County. Colerain Township's boundaries were described as:
Bounded by Dublin Township as above, by the Provencial Line, and Top of Dunnings Mountain so as to join Cumberland & Bedford Townships to the gap of Morrisons Cove from thence to the Mouth of Yellow Creek joining Barre Township to Strike Sidling Hill. William Parker Constable
Being one of the original townships, the region that started as Colerain would eventually be reduced in size by the removal of Providence Township in 1783 and Southampton Township in 1799. Monroe Township would come out of Providence in 1840. Then, in 1844, the region remaining as Providence Township would be divided into West Providence and East Providence. Mann Township would be removed from Southampton in 1876. And finally, Snake Spring Township would be formed out of West Providence and Wells and Brush Creek townships in Fulton County would be formed out of East Providence when Fulton County as erected in 1850.
The 1767 description does not provide a clear and easily identifiable view of the township. It encompassed the whole region lying between the Evitts Mountain in the west and Sideling Mountain in the east. It stretched from the county / state line in the south to the Johns Branch and Harbor Mountain in the north. It covered a total of four hundred and fifty-three square miles in 1767. With the removal of the various other townships, the township of Colerain has come to encompass a total of forty-two square miles, or roughly one-tenth of the original land mass, in the valley between Evitts and Tussey Mountains.
E. Howard Blackburn, in his History of Bedford and Somerset Counties, Pennsylvania, began his coverage of Colerain Township with the statement: "Colerain was one of the original townships of the county, but the date of its organization, while yet a part of Cumberland county, cannot now be ascertained." Mr. Blackburn apparently did not have the opportunity to study the dockets of the Court of General Quarter Sessions for Cumberland County. In those records we find that during the October Session, on the twentieth day of that month in fact, the frontier region gained by the Province of Pennsylvania in the Albany Purchase of 1754 was divided into five townships, of which Coleraine was one.
The township was named for the county seat of County Derry in Northern Ireland.
