Although the Coast Prairie is a sea bottom of very modern uplift, it appears already to have suffered a slight movement of depression, for its small rivers all enter embayments; the larger rivers, however, seem to have counteracted the encroachment of the sea on the land by a sufficiently active delta building, with a resulting forward growth of the land into the sea.
Ferguson has shown that the sediment is carried away from this area by the set of the currents; probably then it has remained free from sediment whilst the neighbouring sea bottom has gradually been filled up. If so, the thickness of the alluvium is at least 1800 ft., and may be much more.
There is more inorganic nitrogen in the sea near the land than in mid-ocean and there is more at the sea bottom than near the surface; finally, there is more in the later winter than at any other season.
On still other parts of the coast a recent small elevatory movement has exposed part of the former sea bottom in a narrow coastal plain, of which some typical harbourless examples are found in Oregon.
The red-clay is an abysmal formation, occurring in the sea bottom in the deepest part of the oceans.