Phila provides an interesting link to some weird geological formations, largely caused by rapid cooling of volcanic liquids, forming columnar basalt structures. The fissures and gaps between the columns occur as the lava (or magma1) contracts.
This drove me crazy as I tried to remember some similarly weird structures that exist somewhere on the Jamaica mainland. Although I didn't quite find the picture I had imagined once seeing, I deduced that it probably involved erosion of limestone and not volcanic activity. China (see right) remains the primo place for finding these strange formations, often referred to as karsts or lapiaz. In simple terms, years of rain and runoff on top of a soluble limestone bed cause sinkholes and scalloping which show much less regularity than the equally strange basalt columns.
Haiti -- side of a hill
Yugoslavia -- rolling karsts
China -- massive straight up out of the ground
Somewhere -- look closely at the laborer in the pic and a klapiaz
France -- spooky
Mexico -- boulders
Jamaica -- cockpit country. I didn't find exactly what I imagined but this stuff in Jamaica looks fairly impenetrable: here, here, here, here
Grand Cayman Island -- Hell with a devil
Sinkhole.org -- a site dedicated to Florida's sink-holes
Google lapiaz and karsts for more pictures.
1A critical point in my early schooling involved knowing that you call it magma if it resides insides the volcano, and lava when it leaves the orifice. Like a lot of things I learned early on, such as the difference between a whole and a natural number, this fact hasn't gained me any great dividends.