Hello, TheWay. Welcome to EvC.
...the flood doctrine usually states that mountains were formed after the flood.
Actually, there isn't any
the flood doctrine. There have been lots of "theories" about where the water came from and where it went: vapor canopies, ice rings, giant water-filled caverns under the earth. The "mountains formed right after the flood" is one idea among many.
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Wouldn't this account for the seashells....
No, because like the other ideas I mentions, and other creationist ideas like accelerated radioactive decay, changing speed of light, Grand Canyon carved when it was soft sediment, and so forth, it is pretty easily shown to not have happened.
Sudden mountain building would look very different than what we actually do see in the mountains, and, besides, it ends up having to invent totally new physics not to account for evidence, but to try to save a preferred creation myth from the evidence.
In many respects, the Bible was the world's first Wikipedia article. -- Doug Brown (quoted by Carlin Romano in
The Chronicle Review)