It is true that they got there because of the flood.Now this happend because the water had to rise above the highest point on earth so there was no escape from death.So the shells rise with the water because certain organisims can only live so deep in the water,and then when the water goes down they are left stranded on the mountain.
In the mountain. They are
in the rocks, not just on the surface.
As to how oyster beds and coral reefs climb mountains, you have not proposed any details and I'd like to see you try. And having climbed up, as you claim, why couldn't they climb down again?