As more and more sediment was washed down to the deltas they built up solid ground and so the mouths of the rivers advanced in the direction of flow.
(more and more sediment; was washed down; to the deltas) (they; built up; solid ground) (the rivers; advanced; L:in the direction of flow)
more and more sediment they the rivers
was washed down built up advanced
to the deltas
solid ground
L:in the direction of flow