Divisions of the tertiary.  The formations of the Tertiary are grouped in three divisions,—­the Pliocene (more recent), the Miocene (less recent), and the Eocene (the dawn of the recent).  Each of these epochs is long and complex.  Their various sub-divisions are distinguished each by its own peculiar organisms, and the changes of physical geography recorded in their strata.  In the rapid view which we are compelled to take we can note only a few of the most conspicuous events of the period.

Physical geography of the tertiary in eastern north America.  The Tertiary rocks of eastern North America are marine deposits and occupy the coastal lowlands of the Atlantic and Gulf states (Fig. 260).  In New England, Tertiary beds occur on the island of Martha’s Vineyard, but not on the mainland; hence the shore line here stood somewhat farther out than now.  From New Jersey southward the earliest Tertiary sands and clays, still unconsolidated, leave only a narrow strip of the edge of the Cretaceous between them and the Triassic and crystalline rocks of the Piedmont oldland; hence the Atlantic shore here stood farther in than now, and at the beginning of the period the present coastal plain was continental delta.  A broad belt of Tertiary sea-laid limestones, sandstones, and shales surrounds the Gulf of Mexico and extends northward up the Mississippi embayment to the mouth of the Ohio River; hence the Gulf was then larger than at present, and its waters reached in a broad bay far up the Mississippi valley.

Along the Atlantic coast the Mesozoic peneplain may be traced shoreward to where it disappears from view beneath an unconformable cover of early Tertiary marine strata.  The beginning of the Tertiary was therefore marked by a subsidence.  The wide erosion surface which at the close of the Mesozoic lay near sea level where the Appalachian Mountains and their neighboring plateaus and uplands now stand was lowered gently along its seaward edge beneath the Tertiary Atlantic to receive a cover of its sediments.

As the period progressed slight oscillations occurred from time to time.  Strips of coastal plain were added to the land, and as early as the close of the Miocene the shore lines of the Atlantic and Gulf states had reached well-nigh their present place.  Louisiana and Florida were the last areas to emerge wholly from the sea,—­ Florida being formed by a broad transverse upwarp of the continental delta at the opening of the Miocene, forming first an island, which afterwards was joined to the mainland.

The Pacific coast. Tertiary deposits with marine fossils occur along the western foothills of the Sierra Nevadas, and are crumpled among the mountain masses of the Coast Ranges; it is hence inferred that the Great Valley of California was then a border sea, separated from the ocean by a chain of mountainous islands which were upridged into the Coast Ranges at a still later time.  Tertiary marine strata are spread over the lower Columbia valley and that of Puget Sound, showing that the Pacific came in broadly there.