The marginal rises and continental shelves prevent this cold bottom water from penetrating into the depths of the enclosed and fringing seas.
The value per acre of land, which exceeds 48 in the departments of Seine, Rhne and those fringing the north-west coast from Nord to Manche inclusive, is on the average about 29, though it drops to 16 and less in Morbihan, Landes, Basses-Pyrnes, and parts of the Alps and the central plateau.
Farther east the sea was interrupted by the still existing land-connexion between Tasmania and Victoria; but beyond it, the marine deposits are found again, fringing the coasts of eastern Gippsland and Croajingolong.
Barrier reefs are rare; fringing reefs are numerous, except on the east side, which is nearly free, and there are many small isolated reefs and volcanic banks among the islands.
Whole forests, vast quarries of granite, and hills of gravel were used in fringing the water margins, constructing wharves, piers and causeways, redeeming flats, and furnishing piling and solid foundations for buildings.