Bending And Breaking of The Rocks
Perhaps the greatest geologic contrast between these two closely adjacent parks lies in their different geologic structure—the kind and amount of bending and breaking of the once nearly flat lying strata. Consolidated rocks, particularly brittle types, are subject to two types of fracturing by Earth forces. Joints are fractures along which no movement has taken place. Faults are fractures along which there has been displacement of the two sides relative to one another ([fig. 6]). As noted in the report on Canyonlands National Park ([Lohman, 1974]), the strata there, particularly along the valley of the Green River, are virtually flat lying or have only very gentle dips. Along the Colorado River above the confluence with the Green, however, the slightly dipping strata are interrupted by several gentle anticlinal and synclinal folds ([fig. 5]) and by at least one fault ([fig. 6]). The largest of these folds—the Cane Creek anticline, which crosses the Colorado River north of Canyonlands—has yielded oil in the past and is now yielding potash by solution mining of salt beds in the Paradox Member of the Hermosa Formation.
COMMON TYPES OF ROCK FOLDS. Top, Anticline, or upfold; closed anticlines are called domes. Bottom, Syncline, or downfold; closed synclines are called basins. From Hansen (1969, p. 31, 108). (Fig. 5)
In strong contrast to Canyonlands, Arches National Park contains three northwesterly trending major folds and is bordered on the southwest by a fourth. The largest and most important are the collapsed Salt Valley and Cache Valley anticlines, which separate the two most scenic groups of arches and other erosional forms—Eagle Park, Devils Garden, Fiery Furnace, and Delicate Arch on the northeast, and Klondike Bluffs, Herdina Park, and The Windows section on the southwest. Farther southwest is the Courthouse syncline, containing the attractive group of erosional forms called Courthouse Towers ([fig. 1]). Finally, near the southwest edge of the park, is the Seven Mile-Moab Valley anticline (also known as the Moab-Spanish Valley anticline), whose southwest limb is cut off by the Moab fault (figs. [7], [23]). The folds just named and the sharply contrasting geologic structures of the two parks are well shown on sheet 2 of the geologic map of the Moab quadrangle (Williams, 1964), and the geologic formations are shown in color on sheet 1.
COMMON TYPES OF FAULTS. Top, Normal, or gravity fault, resulting from tension in and lengthening of the Earth’s crust. Bottom, reverse fault, resulting from compression in and shortening of the Earth’s crust. Low-angle reverse faults generally are called overthrusts or overthrust faults. In both types, note amount of displacement and repetition of strata. Displacements may range from a few inches or feet to many thousands of feet. From Hansen (1969, p. 116). (Fig. 6)
PARADOX BASIN, in southeastern Utah and southwestern Colorado, showing the extent of common salt and major potash deposits in the Paradox Member of the Hermosa Formation, and the salt anticlines. Adapted from Hite (1972, fig. 1B). (Fig. 7)