OCR Text |
Show Park Service "Report By Combiner! Feelers if Itetere Canyons from in many ways. wa-ys. About the only thing common com-mon to all canyon is that they th-ey have been deepened try running run-ning woter, ice, or both. There The-re aremony contributing factors fac-tors to canyon development. The depth of the canyon, the slope of its walls, the course cf the canyon, the structural grain or joint systems in the rocks are all important parts of individual canyons and systems sy-stems of canyon. Each part, must be explained if the origin ori-gin of the canyon is to be : known. Streams deepen their courses cour-ses by downcuttinig. A stream may downcut through the rock ro-ck at the same rate that the rock is uplifted. If the rate of downcutting is equal to the rate of uplift, the stream will deepen its course and develop a master canyon foi the region. If the stream traveled in a winding, sinuous course before the land began to uplift, the resulting master mas-ter canyon will be a winding, sinuous one. Good examples are the canyons of the Colorado Colo-rado river and San Juan Rivers. While this is a simplified sim-plified version of master stream and canyon development, develop-ment, it is an acceptable model for the Colorado river , ... canyon in the Moab area. " But what about the miles' of side canyons that drain into the Colorado river. Are these all produced by ancient streams stre-ams downcutting as the land uplifted? The answer is No. . Once a major canyon develops, deve-lops, the stream carving it is able to downcut faster than the intermittent tributary streams which feed into the major canyon. Since the tributary tribu-tary streams can't keep pace with the downcutting of tne master stream, their mouths are left "hanging" on the canyon can-yon walls above the major stream. A feature called a nick-point generally develops at the "hanging mouth" of the tributary drainage. You know what a nick is: a gouge in a piece of furniture furni-ture or a dent in the car fender fend-er are nicks. Applied to rocks, a nickpoint is a sharp, steep drop in a canyon course. This drop froms a cliff face or "jump". A walk in many of our local canyons will reveal these nickpoints. When you meet them, your route is frequently blocked. The cliff or "jump" is too high and steep to climb, as are the canyon can-yon walls on either side. Many Ma-ny times the canyon narrows, elmost as a funnel. During flooding, there is an impression impress-ion of squeezing energy as the water rushes over the nick-point. nick-point. These nickpoints are. one key to much of the canyon erosion around us. If you look at the profile of a tributary side canyon you see a very interesting thing. The profile is not a smooth, downward sloping line from the headwaters to the mouth. Instead, it is a series of steps. Each step in the stairway leading to the master river is one of these nickpoints. " By headward erosion, undercutting undercut-ting of soft layers, collapse cf overlying harder rock layers. , these, stair steps- work away :: from-, the master stream to-, to-, wards the headwaters of the tributary drainages. The side canyons are lengthened, widened wid-ened and deepened by this process. The major river or master canyon provided the initial opening in the ground for the begining of this lateral erosion. But, after that, it has little to do in the development of the canvon topography. |