I am of the mind that whatever was built was (as GraceG said) was clustered around the rivers and sea shores, and subsequently inundated. Witness the Chesapeake Bay, which is actually the Susquehanna River basin.....flooded. Right before the river enters the bay, it cuts through a gorge (part of which is flooded behind the Conowingo Dam). There is a sign along one of the trails that says that the gorge was cut by the meltwaters from the glaciers. It must have been fantastic, as the sides of the gorge are granite.
Think the equivalent of the New River Gorge in West Virginia... Areas barely out of the water now were like the badlands terrain along the Little Missouri River today.