Near Iowa City in Johnson County, Iowa — The American Midwest (Upper Plains)
An Ancient Sea
Photographed By Connor Olson, July 25, 2020
1. An Ancient Sea Marker
Inscription.
An Ancient Sea. . Although the huge 1993 increase in water flow changed the landscape at the Dam site and caused flooding downstream, catastrophic flooding was avoided. The Iowa River returned to normal flow over the next few months, and the Dam itself remained structurally sound. By August, a by- product of the event appeared as the waters receded. The flood had opened a window to Earth's ancient geologic past. The scoured rock surfaces below the Spillway revealed clear evidence of the shallow tropical seas that existed here 375 million years ago, during the Devonian Period.
Although the huge 1993 increase in water flow
changed the landscape at the Dam site and caused
flooding downstream, catastrophic flooding was
avoided. The Iowa River returned to normal flow
over the next few months, and the Dam itself
remained structurally sound. By August, a by-
product of the event appeared as the waters
receded. The flood had opened a window to
Earth's ancient geologic past. The scoured
rock surfaces below the Spillway revealed
clear evidence of the shallow tropical
seas that existed here 375
million years ago, during
the Devonian Period.
Location. 41° 43.303′ N, 91° 31.932′ W. Marker is near Iowa City, Iowa, in Johnson County. Marker is on Prairie Du Chien Road Northeast. Touch for map. Marker is at or near this postal address: 2850 Prairie Du Chien Rd NE, Iowa City IA 52240, United States of America. Touch for directions.
Credits. This page was last revised on June 7, 2021. It was originally submitted on June 6, 2021, by Connor Olson of Kewaskum, Wisconsin. This page has been viewed 141 times since then and 14 times this year. Photos:1, 2. submitted on June 6, 2021, by Connor Olson of Kewaskum, Wisconsin. • Mark Hilton was the editor who published this page.
