The geological formation of the cave in which Cliff Palace is situated is similar to that at Spruce-tree House, consisting of alternating layers of hard and soft sandstone, shale, and even layers of coal. Both canyons and caves appear to have been formed by the same processes. In past ages the elements have eroded and undermined the soft layers of sandstone or shale to such an extent that great blocks of rock, being left without foundations, have broken away from above, falling down the precipice. Many of these great bowlders remained on the floor of a cave where it was broad enough to retain them. The surface of the roof arching over Cliff Palace cave is perhaps smoother than that of Spruce-tree House. The progress of cave erosion was greatly augmented by the flow of water from the mesa summit during heavy rains, as hereinafter described. [Illustration: FIG. 1.--View down Navaho Canyon.] To understand the general plan of Cliff Palace it is necessary to take into consideration the method of formation and the configuration of the cave floor on which the ruin stands. This cave, as already stated, was formed by erosion or undercutting the softer rock at a lower level than the massive sandstone, leaving huge blocks of stone above the eroded cavities. Naturally these blocks, being without support, fell, and in falling were broken, the larger fragments remaining on the floor practically in the places where they fell, but many of the smaller stones were washed out of the cave entrance, forming a talus extending down the side of the cliff. The floor of the cave was thus strewn with stones, large and small, resting on the same general level which is that on which the foundations of the buildings were constructed. The level of the cave floor was interrupted by the huge blocks of stone forming its outer margin; and the buildings constructed on these fallen rocks were lofty, even imposing. The talus composed of fallen rock and débris, piled against the canyon side in front of these buildings and below these huge blocks of stone, extends many feet down the cliff in a gradual slope, covering the terraced buildings and burying their retaining walls from sight.[16] A great part of this talus is composed of fallen walls, but considerable earth and small stones are contained in it, probably precipitated over the rim of the cave roof by the torrents of water which sometimes fall during heavy rains. It is probable also that the foresting of the talus has been due more or less to bushes and small trees washed over the cliff from the mesa summit. [Footnote 16: Access to Cliff Palace from the bottom of the canyon, although difficult, is possible, and a pathway might be constructed down its sides or along the top of the talus to several other cliff-dwellings. In the vicinity of Cliff Palace there are at least 20 ruins, large and small.]