The rocks are usually globular or crenellated, that is resembling a brain, or perhaps a head of cauliflower. The outer surface is a rusty color, but when cracked or scraped, the inner structure resembles cast iron. The stones are rather brittle, and flake easily under pressure of common pliers.
Fine sand permeates the structure of the rocks.
These are not industrial waste or slag, as they are firmly lodged in sandstone on a hillside that has never been developed. As the bluff erodes, rusty spots slowly appear, which eventually reveal these rocks. We're really curious as to what these may be. Any help would be most appreciated.