Green Gabbro

Julian is hosting this month's Accretionary Wedge, and wants us all to discuss a geologic event that's significant to us personally. (Well, technically, he asked for the event that is most significant, but I love all my pet geologic events equally, so there, nyah.)

The nearly record-setting floods on the Mississippi River this spring have brought back memories of the summer when it just. kept. raining. During a rare break in the weather in July 1993, my mother took me, my sister, and friends up to the local Army Corps flood control structure, and the Coralville Lake behind it; of course, we got rained out. On the way back, though, we passed the dam's emergency spillway... and it was spilling!

The lake didn't fall back below the spillway for another month. During the flood, the Iowa River swept off fifteen feet of topsoil and flood plain deposits, and began cutting in to the 375 million year old sea floor underneath. When the water finally receded, it exposed a whole ecosystem, corals and crinoids and brachiopods with the occasional primitive fish.

The path from the spillway to the river has been turned into a paleontological exploratorium. Some of the better fossils are marked, but most of them are left for visitors to discover themselves with the aid of fossil identification guides posted near the parking lot.

Watching the river flow over the dam didn't turn me into a geologist, but seeing the river swallow so many roads and buildings that summer did give me a deep appreciation of what it means to live on a flood plain. Lots of people do, in Iowa City; most of the time they enjoy lovely river views.