Last but far from least among the factors responsible for the grandeur of Arches National Park and the Plateau in general is the desert climate, which allows one to see virtually every foot of the vividly colored naked rocks, and which has made possible the creation and preservation of such a wide variety of fantastic sculptures. A wetter climate would have produced a far different, smoother landscape in which most of the rocks and land forms would have been hidden by vegetation. On the Plateau the vegetation grows mainly on the high mesas and the narrow flood plains bordering the rivers, but scanty vegetation also occurs on the gentle slopes or flats.
The combination of layers of sediments of different composition, hardness and thickness, the bending and breaking of the rocks, and the desert climate, has produced steep slopes having many cliffs, ledges, and fins with generally sharp to angular edges, rather than the subdued rounded forms of more humid regions.
Among the questions commonly asked by visitors are, "How do arches form?", "Why are some openings called windows, others arches?", "What is the difference, if any, between arches or windows and natural bridges, such as those at Natural Bridges National Monument?", and "How many arches are there in Arches National Park?" Before taking up the origin and development of arches, I shall attempt to explain the differences between the three types of natural rock openings named above and comment upon the number of arches.
[Illustration: INDEX MAP, showing localities where most of the photographs were taken. Arrows point to distant views. Numbers refer to figure numbers. (Fig. 13)]
I believe most geologists and geographers are in general agreement with Cleland (1910, p. 314) that "a 'natural bridge' is a natural stone arch that spans a valley of erosion. A 'natural arch' is a similar structure which, however, does not span an erosion valley." According to this definition, Natural Bridges National Monument includes three true bridges, whereas all the larger rock openings in Arches National Park with which I am familiar are properly termed "arches," but some are called windows. If we were to distinguish between arches and windows, we might say that arches occur at or near the base of a rock wall, as do the doors of a house or building, whereas windows are found well above ground level. This distinction was not followed in naming the rock openings in the park, however; for example, Tunnel Arch (fig. 14) is considerably higher above the ground than North Window (figs. 37, 38) or South Window (fig. 39).