During the earlier half of the last period (the Cretaceous) of the Mesozoic era, sea water spread from Mississippi northwestward to the site of Denver and southward over Texas and much of Mexico. At the same time much of the western margin of the continent from Alaska to California was submerged. All the rest of the continent was land. During this time sediments accumulated on low lands just east of the site of the present Rocky Mountains, and also east of the Appalachians, as proved by the numerous fossils of land plants found in these deposits. [Illustration: Fig. 43.--Map Showing the general rotations of land and water in North America during later Cretaceous time, several million years ago. Lined areas represent land; vertical lines, mainly continental deposits. (Principal data from a map by Willis, published in the Journal of Geology.)] As Cretaceous time went on the marine waters gradually spread until the whole Atlantic and Gulf coastal plain regions from Long Island, New York, to Mexico became submerged under marine water, and a wide arm of the sea, or great mediterranean, spread from Texas north to the mouth of the Mackenzie River. The Gulf of Mexico was thus directly connected with the Arctic Ocean. This great interior sea was nowhere connected with the Pacific Ocean, though portions of the Pacific border of the continent were submerged. This vast interior sea was not only the largest of any which reached well into the continent since the Mississippian period of the Paleozoic era, but it was the last body of marine water which ever extended well into the continent. It should be stated that the later Cretaceous was also a time of unusually widespread submergence of the continents, when most of southern Europe and southeastern Asia, as well as about one-half of both Africa and South America were submerged. Over much of the site of the Rocky Mountains during the late Cretaceous there were low lands receiving continental deposits, and extensive marshes supporting prolific vegetation were common. Much of this vegetable matter became buried, and has since been converted into workable coal. The maximum thickness of strata accumulated during all of Cretaceous time over the Atlantic coastal plain area was about 1,700 feet; over the Gulf coastal plain region fully 7,500 feet; over the western interior 10,000 to 15,000; and over parts of the Pacific border 25,000 to 30,000 feet, as in California. The last-named figures are truly phenomenal, representing a thickness about equal to the total thickness of all the strata accumulated during the whole Paleozoic era (seven periods) and piled up in the Appalachian Mountain region. This great deposit of strata of mostly early Cretaceous Age is readily accounted for when we realize that these sediments, which accumulated in the marginal sea bottom, were derived from the very rapidly eroding, newly formed lofty Sierra Nevada Range.