The midwest US used to be the bottom of a sea. Did the sea level fall ? Or did the land rise?
Yep. It is a dynamic system, ever changing over the long haul--which makes the enviro's attempts at static preservation all the more ridiculous.
My bet is that the land rose, considering the craton has been squeezed from the right by the expanding Atlantic plate and pushed from the left by the East Pacific Rise.
Just the weight of the last Ice Age's ice melting off let the continent rise a little (isostatic rebound), just not as much as the volume of water which melted off raised the sea level.
There has been plenty of sediment going into the middle, too, from the Rockies and the Appalachian/Alleghany mountain chains--not to mention all that bulldozed down from up north by the glaciers and redistributed by wind and water.