You used to find them everywhere in Cornwall when the Cornish were in charge. These granite stone blocks were laid over rovers and streams to provide crossing pints. Most were not more than eight feet long and accommodated single file traffic but they stayed for centuries and kept a lot of feet dry. A few were set in parallel and much wider to give riders way over rushing streams in winter when water levels were high. Cornwall had a lot of streams so these ancient bridges were on most pathways between settlements.
I keep finding them though there are very few left. Bumped into one today on my walk. Saw another two weeks ago which was, like the one today, two large granite blocks as the ford was wider than normal.