[Bernard Marx]

at the very least it probably shows a debris line from a tsunami

Maybe, but being a rock guy I perked up at this: "How were these huge stones of Laterite and Deccan Trap variety transported to the coast?"

I'm not that familiar with the occurrence of those rock types in the immediate area. If they indeed are "foreign" it raises interesting possibilities.

[C0y0Te]

They tried to hold back the sea.