How is the geological birthday of a mountain range determined? In the preceding paragraph we stated that the Appalachians were folded and born out of the sea about the close of the Paleozoic era. This is readily proved by calling attention to two facts. First, the youngest strata involved in the folding are of Permian, or late Paleozoic Age in the geologic column, as proved by their fossil content, etc., and obviously the folding must have taken place after they had been deposited. Clearly, then, the folding could not have taken place before very late Paleozoic time. Second, the oldest strata resting upon the folded rocks are of early (not the very earliest) Mesozoic Age, and these strata are somewhat tilted but not folded. Obviously, then, the folding must have occurred before the nonfolded strata were deposited, which means that the folding must have been essentially completed in not later than early Mesozoic time. Or, in the case of the Rocky Mountains, we know that strata were there folded late in the Mesozoic era or very early in the Cenozoic era, because folded rocks as late in age as late Mesozoic (Cretaceous) have resting upon them, in some places, nonfolded strata of early Cenozoic (Tertiary) Age. The figure clearly shows how the Ordovician strata must have been folded before the next (Silurian and Devonian) strata were deposited upon them in southeastern New York. [Illustration: Fig. 25.--Diagram illustrating the topography and folded structure of the Appalachian Mountains west of Harrisburg, Pennsylvania. The valleys have been etched out of belts of weak rocks, while outcropping resistant rocks stand out to form ridges. Note the course of the Susquehanna River across the mountain ridges, this being a "superimposed river" (see text, p. 233). (Drawn by A. K. Lobeck.)] [Illustration: Fig. 26.--Only slightly tilted strata of Silurian and Devonian ages resting upon folded strata of Cambrian and Ordovician ages in an east-west section across the Catskill Mountains and Hudson Valley of New York. The folding took place at the time of the Taconic Mountain Revolution toward the end of the Ordovician period. (Drawn by the author.)] As already suggested, however, folding is not the only method by which mountains are formed. Many ranges are either entirely due to the tilting of earth blocks by faulting or fracturing of the earth, or their present altitude, at least, is a direct result of faulting. Such may be called block mountains. They are wonderfully represented by the various north-south ranges rising some thousands of feet above the general level of the Great Basin region of Utah and Nevada. These ranges are, in short, somewhat eroded edges of approximately parallel-tilted fault blocks lying between the Sierra Nevada Range and the Wasatch Range. In southeastern Oregon a series of nearly parallel block mountains, up to forty miles in length and over 1,000 feet in height, show very steep eastern fronts only slightly modified by erosion.