In our trip across the mountains it would be strikingly evident that the strata are no longer in their original horizontal position, as they were piled up layer upon layer, but that they have been notably disturbed and thrown into folds (Plate 7), large and small, some masses of the strata having been bent upward (anticlines) and others downward (synclines). Such folded structures could have been developed only by a great force of lateral compression in the earth's crust within the zone of flowage. Under compression the strata were mashed together, notably bent into curves (folds), and more or less upraised. It would also be readily observed that the main axes of the folds extend essentially parallel to the main trend of the mountain range, thus proving that the force of compression was exerted at right angles to the trend of the range. [Illustration: Fig. 24.--Diagrammatic sections illustrating the development of a typical folded mountain range. Upper figure: A, the old land eroded to furnish sediments deposited under the adjacent sea at C. Middle figure: strata (C) folded as they would appear if unaffected by erosion, and a down-warp (B) between A and C. Lower figure: condition after profound erosion, and filling of B with sediment. (Drawn by the author.)] Using a biological analogy, a brief history of a typical folded mountain range may be stated as follows: First, there is the prenatal or embryonic stage when the materials of the range are gathering, that is when the sediments are piling up layer upon layer relatively near shore on a sinking sea bottom. Next comes the birth of the range when, due to the great lateral compressive force, the strata are thrown into folds and forced to appear above sea level, the range thus literally being born out of the sea. During the next, or youthful stage, the range grows (with increasing altitudes) because of continued application of the compressive force. Even during the youthful growing stage weathering and erosion attack the range and tend to reduce it. Then comes the stage of maturity, when the upbuilding (compressive) force and the tearing down (erosive) force about counterbalance each other. At this time the range has reached its maximum height and ruggedness of relief, with ridges and valleys higher and deeper than at any other time. The old-age stage sets in when the upbuilding power wanes or actually ceases, and erosion dominates or reigns supreme. Slowly but surely, unless there be a renewal by an upbuilding power, the range is cut down until little or nothing of it remains well above sea level, or, in other words, until a peneplain is developed. This last stage may truly be called the death of the range. Usually, however, some local portions of the disappearing range, which are more resistant or more favorably situated against erosion, are left standing to at least moderate heights above the general level of the plain of erosion.