Pizza Ranch Pebbles; a Rapid City mystery


   Next to the Pizza Ranch, east of Walmart, is an empty lot with piles of rock.  But this is no ordinary rock.  Many are round, spherical, and up to 6 feet in diameter.  Some are cracked open, and inside is a pattern of calcite like a spider web.  Very strange. How were these formed?  Where did they come from?  Is this the only place where they are found?  Are there more?  I don't know how many there are in the area, but a friend sent me a link about concretions that probably explains what they are.

piles of strange rocks near Pizza Ranch

spherical rocks up to 6 feet in diameter

spherical rock, about 3 feet in diameter

inside of spherical rock

inside spherical rock