THREATENING ROCK CRASHES

Threatening Rock, 30,000-ton block of sandstone in New Mexico's Chaco Canyon National Monument, toppled over, during the afternoon of January 22, and caused considerable damage to a portion of the famous prehistoric apartment house, Pueblo Bonito. Big chunks of the 100-foot high Rock, some of them as large as a small residence, fell into the 800-room pueblo, demolishing much of the stabilization work that a Navajo Indian CCC Unit had recently completed on the 1,000-year-old structure. A 50-year-old Navajo, sharing the belief of some of his tribesmen that the world would end when the Rock came down, crouched in his tent and cried.

Present plans of the National Park Service are to leave the Rock as it fell, and to resume stabilization work on the pueblo.

Threatening Rock was a detached segment of the cliff wall, near the base of which Pueblo Bonito was constructed, probably between the 10th and 12th centuries. The prehistoric Pueblo Indians apparently feared it might crash, for they built masonry buttresses to protect the base of the huge block. In recent years the Rock had been moving in fractional inches, sometimes inward, sometimes outward. Heavy rains and snows in early January are believed to have undermined it.

THREATENING ROCK. Before and after it fell.
Circular areas in the ruins of Pueblo Bonito are kivas, or ceremonial chambers.

A 15-ton boulder fell from near the top of the Rock on the night of January 21. Its crash rattled windows in the residence of Custodian Lewis T. McKinney, 600 feet away. McKinney climbed the cliff the following noon and found that since the last measurements, on December 23, the Rock had moved outward 10 inches and settled 4 inches, as well as moving west 14 inches. While he was measuring he could hear the Rock popping and cracking, and he could feel it grating and grinding.

The Rock fell at 3:24 P.M. The slab leaned out about 30 or 40 feet from plumb, and settled sharply. Rocks from the top broke loose and were propelled into the ruin. Some fragments were thrown 360 feet from the Rock's original base. The lower two-thirds then pivoted on its outer edge and fell down the slope toward the ruin. The whole mass broke into many fragments, and an avalanche of rocks catapulted down the slope and into the walls of the back portion of Pueblo Bonito. The highest walls of the ruin were not damaged. A 1,000-pound boulder was hurled 50 feet over a cement mixer at the side of the ruin,

Approximately one-fourth of the original volume of Threatening Rock now fills 21 damaged rooms of the pueblo. The other three-fourths lie in an imposing jumble of huge rocks between the ruin and the Rock's original base. The debris portrays in graphic fashion the conclusion of a story of fear and anxiety which had its beginning a millennium ago.