Have We Got Concretions

It seems that by far the most popular part of the Geology Forum is people asking for help identifying their rocks, and the most popular of those, in turn, seem to be people with concretions. It has always been that way, even before forums and computers. The earliest geological thinkers had to figure out what to do with the shapes found within rocks. If you know nothing about rocks, how do you know what's a crystal, what's a fossil, what's a concretion, what's a vug or a geode or an amygdule? How do you know which came first? The first geologist, Nicholas Steno, made his name with an essay on "solids contained within solids," and it was a great advance. Concretions are solids that are younger than the rocks they inhabit. Come see a bunch of them in this picture gallery.

Concretion photo courtesy "bueuwe" in the Geology Forum