Sheep crossing?

Found the strangest thing one day in the wilderness. I wandered off the trail a bit, mostly out of boredom, and I found the stream that ran along side the trail just disappeared at one point into the ground. The closer I looked I realized it wasn't natural. Someone had laid huge flat stones over a section, maybe 20-30 feet or so.
 
Then at the other end, it came out of a little channel, since the grade sloped downward there. I was really amazed at who and why someone would do that. It seemed logical that is was a sort of crossing, probably for animals since given the stone walls that abound there, it was most likely fields probably a hundred years ago, maybe less.

Then across the stream, maybe another 30 feet was the ruins of some sort of foundation into the side of the hill, with the entrance clearly on the inside of the hill. That made me think it was probably a pen for the animals, but it was fairly small and not too well reinforced. Too small for cows, I think, but maybe not for sheep. But I'm not sure because I think sheep would be more sure-footed and less likely to need a smooth walkway across the rather rugged stream. Time to do some more research.

This enclosure seems too small for cattle but maybe might have adequately sheltered sheep from bad weather, when once it was covered and more built up.