Architecture in general reached its highest development in Chaco Canyon, and there was real beauty as well as solidity of construction. The walls were massive, although there was a decrease in thickness with succeeding stories, as the weight resting upon them was reduced. The most distinctive type of masonry consisted of a center portion of stone and adobe or rubble, faced on two sides by a veneer of horizontally laid thin, tabular stones. These are so perfectly fitted together that a knife blade can scarcely be inserted between them. Sometimes this particular type of stone was not available and it was necessary to use more massively bedded stones which had to be dressed to the proper shape, but the masonry was uniformly good. Great beams, stripped of bark and beautifully dressed, were placed across the chambers. Small poles, which were finished with equal care, were placed at right angles to the main beams and so spaced as to form patterns. Over these lay carefully fashioned mats of peeled willow, followed by a cedar splint layer. A thick coat of earth overlay the entire mass, forming a floor for the room above as well as a roof for the one below. The use of big logs, which do not bear the scars indicative of transportation over a long distance, and the common use of willow, which must have been abundant, suggest conditions different from those of today. It is not known with certainty whether there has been a real climatic change. Many believe that, when hoofed animals were introduced by the white man, the grass cover was destroyed, and that this led to the cutting of arroyos which carried off flood waters and lowered the underground seepage and as a result the land became progressively drier, but others believe that there were earlier periods of arroyo-cutting. Although severe erosion did not occur until a later time, it was a process with which the ancient inhabitants of Pueblo Bonito were familiar. Overlooking the Pueblo was a tremendous rock with an estimated weight of 30,000 tons, detached from the cliff and seeming so precariously balanced as to threaten the building. At the foot of the rock the prehistoric inhabitants erected a brace of wood and stone masonry. At first glance it seems a rather pathetic effort, but actually it may not show any ignorance on the part of the ancient Bonitians, but rather a familiarity with certain engineering principles which suggested that protecting the base of the rock would curtail erosion and help to prevent the threatened disaster. The fears of the prehistoric inhabitants were never realized in their time, for it was not until January 22, 1941, that the threatening rock finally fell. It damaged one hundred feet of the back wall of the pueblo and twenty-one adjacent rooms.