We can, perhaps, best convey to the reader some of the main facts and principles regarding folded mountains by considering certain observations which may be readily made in a short trip across a folded range of not too complex kind--for example, across the Appalachian range along the line of the Baltimore and Ohio Railroad, west of Washington, or the Pennsylvania Railroad, west of Philadelphia. It would be most evident that the mountains consist of strata, that is sedimentary rocks, such as sandstone, shale and limestone, which were deposited under water. A few measurements would reveal the fact that thousands of feet in thickness of strata are represented. Careful measurements by geologists have, in fact, shown that the strata were originally piled up layer upon layer to a thickness of 25,000 to 30,000 feet. The fact that they are strata of such great thickness proves that sediments must there have accumulated under water for some millions of years at least. Closer examination of a few good exposures (i.e., outcrops) would further reveal the presence of fossil shells and impressions of marine organisms, thus definitely leading us to conclude that the strata were accumulated under sea water, which, of course, means that the present site of the mountain range was once sea floor. Examination of the rock materials also establishes the fact that the strata are such as were deposited in relatively shallow sea water--that is to say, none are at all of the sort which are now forming under really deep ocean water. Most of the strata represent original sands (and even gravels) and muds which could have accumulated only relatively near shore, that is within about 100 miles, which harmonizes with a statement made in a preceding chapter to the effect that very little land-derived sediment is at present depositing more than 100 miles out from shore. The coarse materials (sands and gravels) could not, of course, be carried many miles out, while many of the strata are covered with ripple marks, thus positively proving their shallow-water origin. We conclude, therefore, that the Appalachian strata are of marine, shallow-water origin. But we have already stated that these strata are at least 25,000 feet thick. How, then, do we reconcile these two seemingly paradoxical statements? All that is necessary is to realize that the floor of the shallow sea, in which the sediments eroded from adjacent land were being deposited, slowly, though more or less irregularly, subsided or sank during the long ages (millions of years) of their accumulation. It would carry us too far afield to really attempt an explanation of this remarkable type of geologic phenomenon, and it must suffice to suggest that, starting with the earth's crust in equilibrium, the very weight of accumulating strata would tend to destroy that equilibrium and so cause subsidence.