Ringing Rocks in Bridgeton, Pennsylvania

If you're going to be anywhere near Bridgeton, Pennsylvania (about two hours west of Philly) you should swing by the Ringing Rocks County Park and pound on some stones, the unique rocks resonate when hit with another stone and sound sort of like a bell.  There's a huge boulder field dull of these musical rocks made from volcanic basalt.  Neatorama has the low-down on other musical rocks and here's more info about Ringing Rocks.