Origin And Development of The Arches 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). As to the number of arches in the park, I might begin by saying that there is no universal agreement as to how large a rock opening must be to qualify as an arch. The pamphlet formerly handed to visitors entering the park proclaimed that “Nearly 90 arches have been discovered, and others are probably hidden away in remote and rugged parts of the area,” but the average visitor probably sees less than a third of this number.