The Gulf border of Mexico and Texas, composed of soft marine sediments, forms a gently sloping plain bordered on the west by a roughened upland, and illustrates the general feature of a recently emerged coastal plain (Fig. 9). The same is true also of the entire coast from Texas to New York, but it happens that a recent movement through this region was of such a nature as to allow the sea to encroach on the land, and the previously excavated stream valleys are now, in part, occupied by the sea. This feature is most marked from the Carolinas to New York (Fig. 6), where there are several great estuaries and drowned river-valleys which extend far into the land. The best examples are Albemarle Sound and Chesapeake and Delaware Bays. The James River channel is submerged as far as Richmond, the Potomac to Washington, the Susquehanna to Harrisburg, the Delaware to Trenton, and the Hudson to Troy. These are typical illustrations of what geographers term drowned river-valleys. They are evidence that the land formerly stood higher than now, was trenched by the rivers that flowed across it, and was then depressed or tilted so as to allow the sea to encroach upon it. The importance of these events in the settlement of North America by Europeans and on the subsequent development of commerce, manufactures, the location of cities, etc., needs only to be suggested to permit the reader to fill in the details for himself. On the Gulf coast and about Florida the later movements of the land have been less than in the region from Albemarle Sound to New York, and estuaries are there absent, with the somewhat marked exception of Mobile Bay. Certain secondary conditions need to be introduced here, but space will not permit of more than a brief presentation of them. Not only have the recent movements of the land been less about the shores of the Gulf of Mexico than in the middle Atlantic region of the United States, but the rivers at the south are in general smaller and less swift than those farther north, and hence are less able to excavate broad valleys. The Southern rivers, such as the Alabama, Mississippi, Rio Grande, etc., are silt-laden and tend to fill their estuaries, while the weaker streams are unable to resist the encroachments of sand-bars and spits built by shore currents, and their mouths have been practically closed. The coast of Texas gives evidence of slight modern subsidence, but the small estuaries formed have, for the most part, been separated from the Gulf by sand-bars. Northward of the middle Atlantic region the recent oscillations of the land continued to increase and reached a maximum about the shores of the Arctic Ocean; on the Pacific coast also there is similar evidence of an increase in the recent earth movements from the south northward.