The niche is roughly 12-18" high . I don't see how it looks like the end of a drain , especially as presently it would empty onto the road (and there is the loch just the other side ) and before the metalled road would presumably have been well covered by hillside . As I mentioned it is a thoroughly isolated structure , and my first thought is that when the road-cut was first made they stopped cutting back at that point precisely because it appeared and could have led to delays if it were investigated .