I've always thought New Yorkers lead a hermit-crab existence, with our dance clubs that used to be banks and our townhouses that used to be stables and our living rooms that used to be factory floors. In the 21st century, many urban lives are lived in shells made by others, for other purposes. But it never occurred to me that this is sometimes literally true, until I came upon the "geologic city" project. Rockefeller Center, I learned there, is made of 340-million-year-old seashells.

Over millions of years, those shells (OK, there were also bits of other sea creatures, like sponges and corals) accumulated at the bottom of a shallow sea, and their calcite eventually formed into limestone. Which substance a highly social mammal then quarried out of the Earth in the now-dry region it called Indiana, and sent to New York City. Where it was turned into a place where said mammal could enjoy the Rockettes around the time of the winter solstice.

Geologic City, a project of the charmingly-named Friends of the Pleistocene, is a field guide to the traces of Deep Time that are visible in New York City's architecture. That red sandstone apartment building that houses your dentist, for instance, is also an artifact of the Triassic. "Materials, colors, and textures that fill our streets are not from the world we inhabit," the GC site explains. "They are from former worlds that existed millions of years ago."

Thanks to Oskar Stein for the link.