The river is not a long one. It rises in the southerly parts of the Southern Uplands in Dumfriesshire, in a number of burns off the hills, principally the Palling Burn whose source is on the southern slope of Collin Hags (836 feet). From here it flows southwards meeting no towns or veritable villages on its course until almost at its mouth.
Passing under the B6357, the Sark reaches the western end of the Scots Dike, at the modern hamlet of Scotsdike. The dyke was created in the reigns of Edward VI of England and Mary Queen of Scots to mark the border, newly settled by agreement, of their respective kingdoms. South from here, the Sark marks the boundary between Dumfriesshire to the west and Cumberland to the east.
The most famous town on the Sark is Gretna Green, best known for its wedding industry. The A74 motorway passes over it just above the rivermouth.
At its mouth, the Sark enters the Solway Firth in a long, tidal channel, and at low tide it joins the Esk, the course of the joint channel traced through the shallow sands for many miles beforr there is open water.
The point of Cumberland opposite the rivermouth is "Sarkfoot Point", part of the Rockcliffe Marsh wetland.
In the days before King James united the two kingdoms in his person this was dangerous ground; the reiver on either side raided in force across the border, secure that the law could not pursue them across the line, and here was the in particular the Sark was at trhe edge of the most lawless part.
Until the settlement between the child monarchs Edward and Mary, the border of the kingdoms was disputed: the English side claimed the Sark as the boundary and the Scots the Esk, to the south. The land in between was known as the Debatable Land, claimed by both and ruled by none, and a haven for the reivers and their private armies.