If you look at the parish/parliamentary boundary between Duxbury/Coppull/Chorley at a point where they meet in Coppull Hall Woods (the old gps on the phone trick) there is something interesting going on.
The boundaries are down the centre of the River Yarrow until it get to the point where the 3 meet, where it wanders some 20m into the woods then back to the river. At the the furthest point there was a 3m pit and next to it this stone (1m x 1.5m). Spotted what looked like a groove, and when cleaned it back there is a 1m cut groove, yes you guessed it, running along the line of the boundary.
A quick 3D model was produced which you can view below. Have a look round the stone and see if there are any other marks.​
The 1830's Tithe map shows the area the stone is in as the 'Course of Track of Stream' so presumable, the boundary was set when before Eller Brook changed course and the stone placed to mark the old course.

Duxbury Coppull Boundary Stone by csdrabble on Sketchfab

The investigation into this boundary stone is part of the larger '3rd Yarrow Bridge' survey which will investigate the whole area round here.