Region III Quarterly

Volume 1 - No. 1

July, 1939


By James B. Hamilton,
Associate Engineer.

One of the interesting features of Chaco Canyon National Monument in New Mexico is "Threatening Rock", so-called because the aboriginal inhabitants of Pueblo Bonito believed it threatened to fall on their 800-room apartment house. This we know, because in about the year 1050, they placed logs and rock walls beneath its overhanging front face to prop it up.

The rock is a mass of sandstone about 100 feet high, 30 feet wide, and 130 feet long. It is about as large as a ten-story building, and weighs around 5,000,000 pounds.

In past geologic ages, the rock was a part of the cliff which walls in Chaco Canyon. For unknown millenniums it has been slowly separating from the cliff behind it, until now it is some five feet from the cliff at its top, and two feet at the ground level. This tilting indicates that the base may extend some fifty feet under ground.

From excavations around Chettro Kettle, a companion ruin a mile up the canyon from Pueblo Bonito, we know that the valley floor was some fifteen feet lower there when the foundations of that structure were laid, than now.

It is certain that Threatening Rock looked considerably more menacing to the Bonitans than it does to us, because it probably towered fifteen or twenty feet higher than now. The walls and terrace they built in front of it, and the sand and soil that have blown in, have reduced its apparent height. Even so, when viewed from a short distance up the canyon, it seems about to fall on the ruins of Pueblo Bonito.

Immediately to the west of the rock against the base of the cliff, is a mass of angular boulders. There is evidence that at one time the rock extended fifty feet or more to the west, and that this portion of the rock fell. Excavations indicate that in falling it crushed some small structures built against the cliff. Possibly it killed people living in them.

Whether by example of this rock fall, or by others along the canyon; or simply because it looked as if it would fall, the ancients expended a great deal of effort to keep its propped up. Exploratory digging in front of it showed that they built up at its base enough material to fill a room with a ceiling height of sixteen feet, width of thirty-two feet and a lenght of 180 feet--a room bigger than many moving-picture halls. Picture doing that with no tools as we know them today, and no power except that of human muscles.

Threatening Rock

They first piled clay against the base of the rock to a height of fifteen or twenty feet, and six to sixteen feet wide; the greater width at the bottom. Then in front of that they built a wall sixteen feet thick of flagstones laid in adobe. On the front face of this they built a veneer of small stones to form a wall looking much like the ruin walls nearby. Then the wedged-shaped space between the sloping bank of clay and the massive stone adobe wall was filled with sand. This construction has been traced to the west and east of the rock, part of it for a total length of 185 feet.

Even all that did not satisfy the Chief Engineer, Mayor, or very possibly, the pottery-making circle that wanted this mass propped up forever. In the base of the rock at the top of the terrace, are two rather large caves. These extend into the rock perhaps a third of its width and at the front are six to twelve feet high. In the mouths of those caves they placed upright logs reaching from the roof of the cave to some as yet unseen foundation. These logs were encased in a wall of stone and mud mortar about five feet wide at the base to two and one-half feet at the top. This last is about the only portion of their work the casual visitor notices, although the top of the veneer facing their terrace can be seen in several places. The centuries have pretty well filled with blown sand the space between what has not fallen down and the ruin walls of Pueblo Bonito. The veneer has been traced by tunneling eight feet under the rock fall to the west. How much further it extends we do not know. It is evident that their construction did not stop the westerly portion of the rock from falling.

Modern engineers smile at the ancients' use of a few small logs and a relatively thin wall of stone and mud to prop up this enormous weight. The mass of material piled in front of, the reck has not much supporting power, as the portion against the rock is composed largely of clay which, under pressure, flows very slowly, like tar--especially if it becomes damp.

It is pretty well believed that the upstanding portion of the rock has been preserved to a certain extent by the work they did. The caves seem to have been formed largely by the blowing away of the more easily erodible portions of the rock. If the Bonitans had not blanketed the gales away from these softer portions of the rock, it is possible this undercutting would be considerably more extensive than it is today.

Were they unduly alarmed? We can say now that they were prematurely alarmed. But we know the rock is falling. Its top is moving toward the ruins--about an inch a year for the last four years.

In 1935, the engineers set a pipe in a horizontal hole in the rock near its top. The top half of this pipe for a few inches was removed. In the cliff opposite, an iron rod was grouted. Its outer end extends into the pipe. On the edge of the trough formed by the half pipe and on the rod, a mark was cut by a hack saw. Now the mark on the pipe and the mark on the rod are over four inches apart. Measurements which are regularly made by the Custodian of Chaco Canyon National Monument, show a little outward movement every month.

Given the problem of a falling rock, what shall we do to stop it? Some say blast it down before it falls on the ruins and more or less wrecks them. Some say block it up, as the ancients did, only use concrete instead of clay, sand, and rock. Others would tie it to the cliff, with rods of steel. The unique archeological exhibit at the base of it must be saved. That is fundamental. Nothing like it exists elsewhere in the world. That being accepted, the first two solutions are ruled out. If we remove the rock, we destroy the visual evidence of why the Bonitans did all that work. If we build a buttress of concrete, we have to remove the work they did. True, we might replace some of it as a veneer over the concrete, but then we would have only a restoration job.

Most engineers who are familiar with the rock believe the best solution is to make use of the discoveries of modern science, and to use steel, air-hammers, treading machines, cement grout, the principle of the lever and the wheel, and many other skills, and processes unknown to the Bonitans. Briefly, it is tentatively planned to drill holes in the back of the rock and face of the cliff near the top, exactly opposite, and to grout steel rods into these holes. The outer ends of these rods would be threaded and these connected by a turnbuckle. As this is turned the rods would be drawn together.

The question of how many rods to use, what size to make them, and how deeply to place them in the rock, has not been settled. Some experiments on pulling rods from sandstone will have to be made so that the engineers can be sure we will use rods enough, long enough and placed deep enough.

Two of the best examples of extinct volcanoes in the Southwest, each with a perfect crater, are now national monuments. Capulin Mountain, in northeastern New Mexico, has a winding road leading to the summit. There is no road to the top of Sunset Crater, one of several cinder cones in the San Francisco Mountains, northeast of Flagstaff, Arizona. The surface of this volcano is covered with fine, loose volcanic ash and cinders.

Sunset Crater is the only one of these Southwestern volcanoes that can be definitely dated. Lava and ashes from one or more of its eruptions completely covered the homes of Pueblo Indians living in the area. Some of these homes have been excavated, and by the tree-ring method, logs used in their construction have been dated at about 860 A. D. Houses built after the eruption are dated after 900 A. D. Hence the last eruption of Sunset Crater is believed to have occurred in the neighborhood of 885 A. D.

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Date: 17-Nov-2005