The first rains and snows which fell upon this ancestral uplift found relatively smooth slopes descending outward from the rim of the central caldera. These rains and melted snows began to drain downhill, finding whatever slight channels or irregularities there were in the surface. As the centuries passed, the little water-channels became gullies, then ravines, trending east and southeast through the Bandelier quadrant, down the natural fall-line of the Pajarito slopes. In less than a million years, the plateau has eroded into its present form and the drainage pattern of canyons radiating from the Jemez ridge and emptying into the Rio Grande has become well defined. Such canyons as Frijoles, then, are the products primarily of water erosion, etched into a one-time smooth slope of volcanic deposits.
During the early years of this erosion process, the caldera itself became a lake, entrapping the runoff of waters within its circle. This body of water eventually found an outlet to the south, through the guarding rim of the basin, and in its outflow began the present system of canyons of the Jemez River. A modern example of a caldera containing a lake is to be seen in Crater Lake National Park in Oregon, but the Valle Grande Lake had nearly six times as great an area.
[Illustration: _Headquarters area in Frijoles Canyon, showing, from bottom upwards, Tyuonyi Ruin, the Big Kiva, the Museum, and the Lodge._]
Many of the almost sheer canyon walls of the monument provide good cross sections of the lava and ash deposits exposed in cliffs several hundred feet high. In simplest form, these cross sections reveal at their base a flow of lava or basalt, overlain by perhaps 200 vertical feet of tuff, and capped by another flow of lava forming the rimrock of the mesa-top. In most places, the alternating layers of lava and tuff were deposited several successive times, variously distributed, and complicated by later faulting and interim periods of erosion, so that the interpretation of the rock layers is not everywhere as simple as in the example given above.
[Illustration: _The Lower Falls of the Rito de los Frijoles._]
One difficulty you may encounter in understanding the makeup of the Pajarito cliffs stems from the very different appearance of the two opposite walls of such a canyon as Frijoles. In the north wall, facing the sun, the cliffs stand bold and somewhat barren; in the south and shadowed wall, there are no prominent cliffs, but rather a rough slope of boulders overgrown with trees and brush. Because of this contrast, it might be difficult for you to realize that the two walls are made up of nearly identical rocks. The difference in appearance is due simply to the difference in exposure. The north wall, hot and dry in the sun and subject to extremes of temperature, has never had a heavy vegetative cover and has eroded into a cliff; the south wall, relatively cool and moist, has been able to support a growth of plants which have held and produced soil sufficient to mask the underlying rocks.