The delta arms sometimes remain blocked with ice the whole year round.
The most fertile soil is the alluvium of the' Delta, deposited during the overflows of the Mississippi.
The Senegal indeed has what is styled an interior delta, but, with the exception of the marigot named, all the divergent branches rejoin the main stream before the sea is reached.
Deposits of the Tertiary period form the basis of more than half the state, extending from the border of the Cretaceous westward nearly to the Yazoo Delta and the Mississippi Bottom, and southward to within a few miles of the Gulf coast.
Besides the delta of the Po and the large marshy tracts which it forms, there exist on both sides of it extensive lagoons of salt water, generally separated from the Adriatic by narrow strips of sand or embankments, partly natural and partly artificial, but havin openings which admit the influx and efflux of the sea-water, and serve as ports for communication with the mainland.