Tabby like "rocks" found in Charleston?

[StephLeigh]

I am a little familiar with the old building material tabby made from oyster shells. Today I came across a bunch of what looked like old rocks. Upon closer inspection I noticed it's more like concrete and has bit of shell and shell imprints. Is it still called tabby if it isn't just oyster shells in the mix? I was about a mile from a colonial site. I would love to know more about this stuff. Hopefully it will lead me to some good finds.