Not exposed in the area but present far beneath the sedimentary cover and exposed in several places a few miles to the northeast are examples of the other two principal types of rocks—(1) igneous rocks, solidified from molten rock forced into or above preexisting rocks along cracks, joints, and faults, and (2) much older metamorphic rocks, formed from other preexisting rock types by great heat and pressure at extreme depths. Igneous rocks of Tertiary age (fig. 59) form the nearby La Sal Mountains. The particles comprising the sedimentary rocks in the area were derived by weathering and erosion of all three types of rocks in various source areas. Arches National Park and nearby Canyonlands National Park are both in the heart of the Canyon Lands Section of the Plateau; therefore, it is only reasonable to wonder why the differences in their general character seemingly outweigh their similarities. First, let us consider the similarities. Both parks are underlain by dominantly red sedimentary rocks, both parks feature unusual erosional forms of sandstone, and both contain beautiful natural arches, although the arches in Canyonlands are restricted almost entirely to the southeastern part of The Needles section and are in much older rocks than those in Arches. To be sure, differences in the rocks themselves play a part in the dissimilarity of the two parks, and these differences are of two types. First, there are lateral changes in the character of the strata, known to geologists as facies changes, brought about by differences in the environment, in the type of materials, and in the mode of deposition even within relatively short distances. Thus, during parts of the Permian Period while sand, later to be known as the Cedar Mesa and White Rim Sandstone Members of the Cutler Formation, was being deposited in the southern part of Canyonlands, red mud, silt, and sand of the Cutler were laid down farther north in Canyonlands (Lohman, 1974, fig. 9), and similar, though somewhat coarser, beds of the Cutler were laid down at Arches (fig. 4). Further comparisons of the rock columns in the two parks show that while limestones of the Rico Formation were being deposited in a shallow sea in the southern part of Canyonlands, additional red mud, silt, and sand of the Cutler were being laid down above sea level in areas to the northeast. The source of the coarser materials was the ancient Uncompahgre Highland, which stood above sea level from Late Pennsylvanian time to Late Triassic time (figs. 7, 59). Although wider and longer, it occupied about the same position as the present Uncompahgre Plateau between Grand Junction and Gateway, Colo. Streams eroded the hard igneous and metamorphic rocks from this ancient landmass and dumped the material into basins to the northeast and southwest. The basin to the southwest, now called the Paradox basin (after Paradox Valley, Colo.), at intervals contained shallow seas and lagoons, which I will discuss later.