John T. Koch defines the specifically Cumbric region as "the area approximately between the line of the River Mersey and the Forth-Clyde Isthmus", but goes on to include evidence from the Wirral Peninsula in his discussion and does not define its easterly extent. Kenneth Jackson describes Cumbric as "the Brittonic dialect of Cumberland, Westmorland, northern Lancashire, and south-west Scotland" and goes on to define the region further as being bound in the north by the Firth of Clyde, in the south by the River Ribble and in the east by the Southern Scottish Uplands and the Pennine Ridge.