Terraforming St Louis

I've just started watching Defiance from the beginning and I'm quite enjoying it. I see a rather largish plot hole which someone may already have commented on ... if, during terraforming, St Louis was covered over by a layer of new ground which hardened before it could crush the city (as explained in S1E2), how come the arch is still as high as it was prior to terraforming? Surely it should be either completely under this supposed shell of new ground, or at the very least poking through it but much, much shorter. Just pondering ...