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By Edwin D. McKee, Park Naturalist. THE Grand Canyon of Arizona has an average width of approximately ten miles within the Grand Canyon National Park, according to measurements obtained by scaling off the distance on a topographic map. The maximum width from rim to rim is found near the Canyon's eastern end where a distance of about eighteen miles exists. To explain this great width the geologist unhesitatingly refers us to erosion - a term which covers that multitude of agencies which everywhere throughout the world are at work destroying the mountains and elevated regions. He tells us that the walls of Grand Canyon simply represent the valley of the Colorado River and he may even go so far as to insist that this canyon is not wide but narrow -- that is, when compared with the valleys of most of our other large rivers. Nearly all of the normal agencies of erosion are involved in the process of widening Grand Canyon. Among these should be mentioned rain, snow, wind, temperature changes, frost, plant action and gravity. Although this knowledge is common to all students of the Canyon, apparently very few of them have given any serious consideration to the relative importance or to the percentage of the entire undertaking which is done by each of these agencies. That wind action plays a comparatively small part in eroding the Grand Canyon has been shown in an earlier article of "Nature Notes"1. This is indicated both by the lack of physiographic evidence and by data on existing meteorological conditions.
The writer is likewise inclined to believe that plant action produces only a negligible amount of Grand Canyon erosion despite the fact that some excellent examples of trees acting as wedges and levers, and of small plants forcing apart and decaying rocks are to be found along the rims and to a less extent down in the Canyon. The general semi-aridity of the region naturally limits the amount of the vegetation so decay effected by it is reduced proportionately. On the other hand, the amount of wind is not excessive so even where trees are numerous their prying action is probably not above normal. It is the comparative lack of a real vegetative covering however, which, in the opinion of the writer, has made running water by far the most important agent of erosion - this, and the nature of the precipitation. It has been aptly said that "the concentrated energy of a single torrential rain (within the Grand Canyon) often does more destruction than a season's rainfall on the densely covered slopes of a humid region". Not only the frequent cloud bursts of the summer rainy season, but also the rapid melting of the heavy winter snows on the Canyon rims produce great destruction to the walls below. An important corollary of running water as an erosive agent is seen in the work of gravity as exemplified by landslides. Although undoubtedly assisted and affected by numerous other forces, the start of most recent slides within the Grand Canyon can be traced either to the direct or to the indirect action of water. To be sure, the usual results of water work are found at the base of every major cliff in the term of talus slopes, but examples of definite slides from this cause are also numerous. Landslides have been classified by Ries2 under four types. Briefly stated these are (1) creeping slides of unconsolidated materials, (2) swiftly moving slides of unconsolidated materials, (3) rock slips and (4) rock falls. The effects of the first two types are to be seen seldom if at all in the Grand Canyon because of the general absence of unconsolidated materials ether than talus. The third type is likewise of little significance here, both because of the semi-arid climate and because of the general horizontal attitude of the strata. Splendid examples of the fourth type, however, are to be found on every hand.
Mr. Emory Kolb who has lived on the rim of Grand Canyon for many years states3 that within the past twenty years no less than seven major rock falls have occurred immediately in front and within sight of his house. Numerous others he has heard roar and rumble during otherwise quiet nights. Above Indian Gardens great piles of debris and rocks -- some as large as houses -- testify to the size of two landslides which occurred during the period of 1910-11. The smallest of these, to the east of the Gardens, occurred during dry weather and involved the entire face of the Redwall cliff. It was both heard and seen by Mr. Kolb. The larger slide started up in the middle Supai sandstone cliff during a storm and came down through the Bright Angel fault area in the Redwall, completely wiping out the trail at Jacob's Ladder.
During a violent storm in December, 1919 a great promontory projecting out into the Canyon just west of Kolb' a studio slid forward and then dropped as a mass. The material finally came to rest far down in the Supai formation. Judging from the descriptions of the preliminary outward slipping in this case, it seems likely that the wetting of the red shales and muds which occur at the base of the major cliff involved (Kaibab limestone) was the cause. If this is true, the landslide should properly be termed a "rock slip" rather than a "rock fall". The courses of three large slides which occurred about ten years ago may be seen in the Supai formation below Grandeur point. At the south end of the Battleship is the debris of a large one which took place at night during the summer of 1931. Again, Ranger Lauzon states that while working on the Bright Angel trail during the winter of 1921 he witnessed slides of various sizes, nearly every day during the thaw of a warm spell in midwinter. Probably the largest and most spectacular rock fall of recent years to occur in this particular area was one which took place below the North Rim at a locality opposite and plainly visible from Grand Canyon Village. Although about nine miles away airline, people on the South Rim could hear the great roar, and see the dust as it rose high in the air. This occurred on a wet summer day and was a remarkable sight. Innumerable other cases of recent landslides in the Grand Canyon might be mentioned, but enough instances have been cited to show the importance of gravity in Canyon widening. So far as can be determined, these falling rocks are usually sections of great cliffs from beneath which soft shales have been undermined. In the nearly horizontal strata of the Canyon walls, vertical joints seem to have much influence on slides. In about twenty records examined, it was also found that most of the movements occurred during thaws following winter snows, although many had been caused directly by summer cloudbursts.
ERRATUM: In Vol. 8, No. 1, page 130 of Grand Canyon Nature Notes, line four under Havasupai basketry should read "and blanket weaving" instead of "and basket weaving". | ||||||
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14-Oct-2011
