Following the Cretaceous period came the Tertiary period, during which the continent assumed very nearly its present outline. During this period, however, as is indicated in Fig. 33, D, the Atlantic border of the United States from New England southward and a wide area about the Gulf of Mexico, were submerged and had deep layers of sediment deposited on them. During the Tertiary, bodies of fresh water became for the first time a conspicuous feature on the land, and large lakes and broad silt-depositing rivers existed particularly in the Pacific mountain region of the United States, and at its close the continent was practically completed as we now know it, but several important oscillations, particularly at the north, have since occurred. [Illustration: FIG. 33.--Maps showing the growth of the North American continent.] With the growth of the continent, briefly outlined above, came greater and greater diversity in its relief, due principally to the upraising of various mountains in a somewhat orderly succession from east to west. The oldest mountains on the continent are the Laurentian Highlands of eastern Canada. Although the region referred to--the one mentioned above as being composed of Archean crystalline rocks--is not now of sufficient elevation or ruggedness to be termed mountainous, it shows in the nature and structure of its rocks that deep erosion has taken place. The inference is that truly great mountains have been removed, but the evidence may also sustain the interpretation that slow upheaval has been accompanied by erosion, and that at no time was the land conspicuously elevated. Next in age after the Laurentian Highlands come the mountains of New England and the maritime province of Canada, which were upraised at the close of the Silurian period. The next great step was the crumpling into folds and upheaval of the rocks in the Appalachian region at the close of the Paleozoic era. The Park and Stony Mountains were upraised at the close of the Mesozoic era, and later came the Sierra Nevada and Cascades, followed by the Coast Ranges. Youngest of all, and in part for that reason the boldest and most lofty, are the magnificent mountains of southern Alaska, with a host of sublime peaks, like Mounts Fairweather, Logan, St. Elias, and perhaps McKinley. The last-named and highest peak of all, however, may be of volcanic origin.