The down-warps or troughlike basins were very favorably situated for rapid accumulation of thick sedimentary deposits because of their position just between large, high land masses which were being vigorously eroded. The sediments derived from the erosion of the young Appalachians were especially abundant because of the vigorous wearing down of the newly formed high mountains. A thickness of from 5,000 to fully 15,000 feet of mostly red sandstones and shales accumulated in these down-warps, the character and great thickness of the strata strongly pointing to gradual down-warping as the deposition of the sediments went on. It is often stated that these strata were formed in estuaries, but, in the northern areas, at least from Massachusetts to Maryland, many of the layers show ripple marks, sun cracks, rain-drop pits, fossil plants, and fossil bones and tracks of land reptiles. Such strata may well have formed in very shallow water, such as river-flood plains or temporary lakes, where changing conditions frequently allowed the surface layers to lie exposed to the sun. [Illustration: Fig. 41.--Block diagram of the region westward from New York City and vicinity, showing the main relief features, the underground relations of rocks of widely different ages, and the relation of the relief to the rock formations. (Part of larger drawing by A. K. Lobeck.)] During the time of the accumulation of the late Triassic strata in the down-warp basins there was considerable igneous activity, as proved by the occurrence of sheets of igneous rock within the body of strata. In some cases true lava flows with cindery tops were forced out on the surface and then buried under later sediments, while in other cases the sheets of molten rock were forced up either between the strata or obliquely through them, thus proving their intrusive character. As a result of subsequent erosion, these very resistant lava masses often stand out conspicuously as relief features. Perhaps the most noteworthy example is the great layer of such intrusive igneous rock, part of which outcrops for seventy miles mostly as a bold cliff forming the famous Palisades of the Hudson, near New York City. During the process of cooling and solidification of the molten mass there was contraction which expressed itself by breaking the rock mass into great, crude, nearly vertical columns, and hence the origin of the name "Palisades." The cliff character of the outcrop is due to the fact that the lava is much more resistant to erosion than the sandstone above and below it. In the Connecticut Valley of Massachusetts a layer of lava several hundred feet thick boldly outcrops, forming the crest of the well-known Mount Tom-Mount Holyoke Range.