The Gulf coastal plain area from Florida through Texas and south through eastern Mexico was largely overspread by the sea during most of Tertiary time, except the latest. During early Tertiary time an arm of the Gulf reached north to the mouth of the Ohio River. Late in the period but little of the Gulf Plain was submerged, and at its close sea water was wholly excluded. On the Gulf Coast the Tertiary strata from 2,000 to 4,000 feet thick are also mainly sands, gravels, clays, and marls. They are commonly rich in fossils, and they show a gentle tilt downward toward the Gulf. Throughout Tertiary time local portions of the Pacific border of the continent were submerged, this having been especially true of portions of California, Oregon, and Washington. In spite of the very restricted marine waters, the Tertiary strata of the Pacific Coast, especially in California, are remarkably thick, 10,000 to 20,000 feet being common, while the maximum thickness is fully 30,000 feet. Such great thicknesses are readily explained when we realize that erosion was notably speeded up by pronounced uplifts resulting from crustal disturbances toward the close of the preceding period, and again in the midst of the Tertiary period itself. To summarize the Tertiary relations of sea and land for North America we may say that only local portions of the continental border ever became submerged, and that, by late Tertiary time, practically the whole continent was a land area. At the close of the period the continent was, as we shall see, even larger than now because the continental shelves of the ocean were then also largely above water. [Illustration: Fig. 45.--Map showing the general relations of land and water in North America during part of the middle Tertiary period. (After Willis, courtesy of the Journal of Geology.)] The whole of the Cenozoic era, including both the Tertiary and Quaternary periods, has been a time of profound crustal disturbances throughout much of the continent, certain of these movements having continued right up to the present time, with positive evidence that some of them are still continuing. These great movements have included notable foldings of strata, uplifts without folding, faulting, and igneous activity, the whole effect having been to greatly increase the general altitude and ruggedness of the continent. In fact, North America is not known ever to have been at once higher, broader, and more rugged than it was very late in the Tertiary, or early in the Quaternary, period. Since that time the only notable change (barring the great Ice Age and its effects) has been a restriction of the area of the continent to its present size by spreading of sea waters over the borders of the continent, that is over the continental shelves. We shall now rather systematically consider the more profound earth changes which have affected the continent, producing the existing major relief features, from west to east.