The last pic shows sign of lifeno, not man made, jus erosion .... what are you referring to in the last picture, all I see is a rock.
Ill have to learn about sandstone concretions. thank you for the info.Are these around Los Angeles area? Most boulders I have seen around San Diego area are granite and the L.A. area is a raised ancient seabed so large sandstone concretions would probably be more likely there. My brother has found fossilized shark teeth on hills in the L.A. area while doing construction.