After further study and identification of this area as a quarry, it appears that this is one of a series of cuts into the bedrock associated with quarry machinery. This one, specifically, may have accommodated one of the posts to which cranes’ tie-rods were tied

Oval cut in bedrock, possibly a garbage pit because it is too large for a posthole. The lack of anthropogenic material may indicate that the pit contained organics rather than anthropogenic waste.