Adams County, carved out of its parent York by the state legislature on January 22, 1800, was the 26th of the Commonwealth's 67 counties. With an area of about 521 square miles, it ranks 44th in size. The law fixed the county seat at Gettysburg.
Several early writers claimed that in quality the soil of Adams County ranked from the worst to the best. Recent soil surveys have tended to confirm that conclusion. There are two major stream systems in the county, both of which have their source in the South Mountain and each of which drains about half of the county. The Conewago Creek flows east through York county and into the Susquehanna River. Marsh Creek flows southeast through Maryland and into the Potomac River.
Although Native Americans, or Indians, used the area as a thoroughfare and hunting ground, there is no conclusive evidence they made it their permanent residence. Europeans began coming into the county in the 1730s and 1740s. Most were Scotch- Irish, English, and Irish in origin. A few were Germans. The existence by 1750 of Presbyterian, Quaker, Anglican, Catholic, Lutheran, and Reformed congregations is evidence of the diversity of the earliest residents.
The first roads through the county, which were established by Lancaster County authority, ran from east to west. Soon thereafter, residents in the northern parts and in Cumberland County gained approval for roads leading south to the newly developed port of Baltimore, through which they marketed their surplus goods. The commercial relations thus established remained close for many years.
In 1783, when the American Revolution ended, the population of the eleven townships in the present Adams County was about 10,900.