Barring certain minor oscillations of level, the region just west of Appalachia was mostly occupied by sea water throughout much of Paleozoic time, and sediments derived from the erosion of Appalachia were laid down layer upon layer as strata upon that sea bottom. In general, the coarsest and greatest thickness of sediments accumulated relatively near the land, while finer materials, in thinner sheets, deposited well out over much of the eastern Mississippi Valley area in the shallow seas which were there so commonly present. By actual measurement we know that the thickness of strata deposited over the site of the Appalachians was at least 25,000 feet. Since these latter strata are mostly of comparatively shallow sea-water origin, as proved by coarseness of grain of material, ripple marks, fossil coral reefs, etc., we are forced to conclude that this marginal sea bottom gradually sank while the process of sedimentation was in progress. Otherwise we cannot possibly explain the great pile of strata of shallow water origin. The very weight of accumulating strata may either have aided or actually caused the sinking of the long, relatively narrow trough. Finally, toward the close of the Paleozoic era, sinking of the marginal sea floor and deposition of sediments gave way to a yielding of the earth's crust by a great force of lateral compression, causing the strata to be thrown into folds well below the surface and more or less fractured in their upper portion. Thus, along the eastern side of the site of the great interior Paleozoic sea, the Appalachian Mountains rose out of what for millions of years had been a long, narrow, sinking sea floor. There was more or less folding from the Gulf of St. Lawrence to central Alabama. Figure 24 diagrammatically represents the principal stages in the history of the Appalachian Range. While the most pronounced earth disturbance occurred through the long Appalachian belt, the whole eastern side of the continent was profoundly affected. Thus the Mississippi Valley area east of the Great Plains was considerably upraised never again to be submerged except along the Gulf Coast, and an eastern interior sea has never since overspread the region which was repeatedly sea-covered during Paleozoic time. CHAPTER XV MEDIEVAL EARTH HISTORY (_Mesozoic Era_) What was the condition of North America during the first or Triassic period of the Mesozoic era, approximately 8 or 10 million years ago? As a result of the Appalachian Revolution the sea was excluded from all the land except along much of the western side from southern California to parts of Alaska. On this western side of the continent the Appalachian Revolution had little or no effect and the Permian conditions continued, essentially without change through the Triassic. The Triassic strata up to 4,000 feet thick are there of typical marine origin. In British Columbia and Alaska there was much igneous activity.