GEOLOGY. The landscape of the Bandelier area is predominantly one of cliffs and canyons; as a visitor to the plateau you will be made conscious of the involved structure and contour of the region in the course of the auto trip over the approach highway. The impression you may get on arrival is of a vast confusion of canyons separated by equally confused mesas and ridges. The topography, however, is not so mixed as first appearance would indicate, for there is a regularity to the pattern of the drainage which becomes apparent from study of a map or aerial photo. The geology, on the other hand, is extremely complex and can be outlined only in general terms in this handbook.
The dominating feature of the landscape is the uplift of the Jemez Mountains, forming the western skyline as one approaches the monument from the east. These mountains are the remains of a great volcano which erupted during the past million years. As seen from a distance, there is very little to suggest a volcano in the profile of the present mountains; only by traveling some 15 miles west of Bandelier into the central valley of the range can the nature of the eruption be visualized. Here is a basin of grassland ringed with forested hills, on a scale so large that its extent is difficult to appreciate. This is the Valle Grande—“great valley” of the Spanish discoverers, who could not have known that they had found one of the largest calderas in the world. Although the Valle Grande now has superficial characteristics of a volcanic crater, there was no single crater here in the days of the eruption—rather a vast dome of a mountain which poured from its flanks such a quantity of lava and other materials that its roof finally fell in. The dimensions of the caldera, a rough oval, are approximately 16 by 18 miles. It is estimated that at least 10 cubic miles of lava and ashes were ejected here to produce the cavity which now exists. The ring of hills around the oval are the remnants of the ancient volcano’s perimeter, which remained elevated after the central areas collapsed.
The volcano, then, played the chief role in fashioning the landscape of the Pajarito Plateau. It provided an uplift of the land at the caldera, by the same means establishing a down-slope from the center outwards, along which the lavas of the eruption avalanched in fire and smoke. Interspersed between the flows of heavy lavas were other avalanches and showerings of volcanic ashes in great depth. When cooled and welded together as they are today, they are called tuff. This process of earth-building went on intermittently for many centuries until the volcano had exhausted its violence and had distributed its many cubic miles of outpourings in encircling deposits around its flanks. With the subsidence of volcanism, the great earth-removing force of erosion became the predominant factor in forming the landscape.