Someone may know this.   
I've finally gotten around to learning "Such a Parcel o' Rogues in a Nation." In the first verse:

Now Sark runs to the Solway sands
And Tweed runs to the ocean
To mark where England's province stands
Such a parcel o' rogues in a nation

I know the two rivers form the boundary between Scotland & England. But the song lines imply to me that Burns was saying this had been fiddled. That is, to increase English territory after the Union, perhaps other, more southern rivers had been renamed or perhaps Sark and Tweed had actually been rerouted. Or maybe something completely different. Anyone know?