A few well-known examples will serve to prove that upward, downward, and differential movements of the earth's crust have actually taken place not only in the remote ages of geologic time, but also that such movements have geologically recently taken place, and that similar movements are still going on. It is very important that the reader thoroughly appreciate the fact that crustal disturbances, often profound ones, do take place, because this is one of the most fundamental tenets of geologic science. Let us consider the case of the Hudson-Champlain-St. Lawrence Valley region. That the whole region was once notably higher (at least 1,000 feet) than at present is proved by the drowned character of the Hudson Valley, in which tidewater extends northward for 150 miles to near Troy. Where the New York City Aqueduct passes under the Hudson River near Newburgh, the bedrock bottom of the old river channel is now about 800 feet below sea level as determined by drilling. This old channel is there filled up nearly to sea level with glacial and postglacial rock débris, which shows that the old channel must have been cut before the oncoming of the ice of the great Ice Age. Before the Ice Age, then, the lower Hudson Valley must have been considerably more than 800 feet higher than at present, because it then contained a river with sufficient current to be an active agent of erosion, carving out the canyonlike valley in the vicinity of West Point. This conclusion is strongly reenforced by the fact that the old valley of the Hudson River has been definitely traced as a distinct trench across the shallow sea bottom for about 100 miles eastward from the entrance to New York harbor. Toward the eastern end of this trench the depth of water is now considerably over 1,000 feet, and thus it is obvious that, preceding the Ice Age, the earth's crust in the vicinity of New York City must have been much higher than at present, so that the Hudson River was able to erode its now completely drowned channel. Somewhat similar evidence has also established the fact that the lower St. Lawrence Valley region was much higher before the Ice Age. It is evident, therefore, that the general Hudson-St. Lawrence Valley region is now notably lower with reference to sea level than it was before the Ice Age. That this was caused by actual sinking of the earth's crust rather than by a rise of sea level is proved by the fact that similar changes of level between land and sea did not take place at the same time even along the Atlantic and Gulf coast of our Southern States.