Probably not man made concrete but different pebbles etc concreted together through time rolling in the water. I've had to break that stuff off of many good solid finds.
Almost looks like glendonite. However, typically not found in the US , but you never know what adventure some of these rocks have made.
Almost looks like glendonite. However, typically not found in the US , but you never know what adventure some of these rocks have made.
