Much of the southern part of the watershed of County Line Branch lies over rock of the Onondaga and Old Port Formation. Some of the southern part of the watershed, however, is over rock of the Keyser and Tonoloway Formation. Most of the middle reaches of the watershed are on rock of the Hamilton Group and the uppermost and northernmost reaches of the watershed are on the Trimmers Rock Formation. There are several soil series in the watershed of County Line Branch. The Chenango-Pope-Holly series is found in a small part of the lowest reaches of the lowest reaches of the watershed, near the stream's mouth. The Hagerstown-Edom-Washington series is found in a much larger area in the southern part of the watershed. The Watson-Berks-Alvira series occurs in the watershed's middle reaches and the Berks-Weikert-Beddington series is found near the headwaters and also on its southwestern edge. There is glacial till in the watershed of County Line Branch. The watershed of County Line Branch has an area of 8.97 square miles. Much of the watershed is in Anthony and Limestone Townships in Montour County, but a significant portion of it is in Lewis Township, Northumberland County. The watershed also occupies part of Turbotville. Its northern edge is on the border of Lycoming County. The headwaters of County Line Branch and their vicinity are mostly forested. However, nearly all of the rest of the it is agricultural land. There is little development anywhere in the watershed. An site called the Pulsifer Site (also known as Site 36Mo79) is a prehistoric archaeological site on a stream terrace of County Line Branch. 32 artifacts have been unearthed at the site, 31 of which are chert flakes. The other artifact is a biface. The material of these artifacts are dissimilar to the bedrock in the area. Some of these artifacts date to the Archaic period and the Transitional period. Agriculture and timbering are done in the upper reaches of the watershed of County Line Branch. A large number of native plants and animals live at the headwaters of County Line Branch. There are also forests at the headwaters, which are part of a large tract of forested land. The Mountour County Natural Heritage Inventory lists the headwaters of County Line Branch as a "locally significant" site. There are 12 tree species at the headwaters of County Line Branch. These include three maple species, two oak species, and two birch species.