Yep old digger, your memory does serve you right. Except theres more! All of the state was part of the interior seaway a long long time ago, around 600 million years. The oldest exposed stuff is in north eastern Iowa and as you travel westward time starts jumping ahead. Around the Ames area the rocks begin to change towards the carboniferous period, a giant swamp that made the coastline to the interior seaway around 360 million years ago. As soon as the bottom of the sea was exposed and dry, it began to loose ground, which is why we don't have any strata with dinosaurs. There is cretaceous rock under north western iowa that potentially has marine dinosaurs, but its not exposed very much.
Best places to look for fossils in iowa seem to be the north east central, to extreme north east. Although they can be found everywhere. Biggest places to look for are bedrock exposures near and down a river.