|Length:||22.6 miles (36.4 km)|
|Meets:||B874, A9, A882, B874, B876|
|Route outline (key)|
The road starts on the B874 at Glengolly, a short distance to the south of Thurso, and heads southwards along a straight single-track road. There's a curve as we go over the top of a hill and we continue on to a staggered crossroads, where we give way to an unclassified road. Continuing south we cross the Far North Line by the isolate Scotscalder station and then enter a scattered wooded area, where the road starts to become more winding.
A T-junction is soon reached in the hamlet of Westerdale, where we TOTSO left to go over the River Thurso and then continue across the Moss to Mybster, where we cross the A9. There's then a detour to the north as we avoid Backlass Hill, after which a few tight bends take us to Watten, the largest settlement on the B870. Here we cross the A882 before going round the edge of Loch Watten to meet the railway line again, which is crossed at an uncontrolled level crossing. Watten station used to be here; its platform is still extant.
A short distance further on we reach Mains of Watten and a T-junction on the B874, which has taken a shorter route to get here than we have. There's a short multiplex to the right after which the B870 turns left to regain its number and continues northeast for a couple of miles. It's not long before the end of the road is reached, at a T-junction on the B876.