Gila River antecedent path

Figure 28 displays the Gila River zig-zag situation. Logic dictates that the Gila's course was established before Basin and Range formation, which is called an antecedent river path, and then as Basin and Range faulting was blocking out the mountains and valleys, the river was stubbornly cutting gorges and maintaining its course. The West has many such situations. Perhaps the most impressive case of river antecedence is the Green River cutting the Canyon of Ladore in Wyoming-Utah across the eastern end of the High Uintas. For a GREAT view of this canyon go to the eastern unit of Dinosaur National Monument along the Utah-Colorado border where you see the southernmost Canyon of Ladore. Somehow the Green River was already flowing south as the Uintas arose in Basin & Range time, and then the Green found the new Colorado River in central Utah in modern Canyonlands National Park and together they re-located an old empty canyon, dropped into it, and formed up the modern Colorado River drainage basin, the final canyon now called 'Grand'.

There is a very small mystery in the Colorado drainage basin involving a tiny fish species called Desert Pupfish. Small populations live now in many isolated springs across the region and survive in very warm saline waters. Their widespread dispersal is likely due to wet rivers everywhere during the ice ages. Several studies have tried to locate their place of origin, such as locating a close species living in the coastal region of Texas - on the wrong side of the continental divide. I have tried personally to answer the question of how a tiny fish may cross the divide, and found a situation near Durango Colorado, a small pond on the divide with streamlets draining into both the Colorado and Rio Grande drainages. Nonetheless, a fossil pupfish was located in Death Valley lakebottom sediments of 7 m.y. age, which suggests they are long-time residents in the Colorado River system. Death Valley is now isolated from the Colorado River, but it seems to have been formerly connected through a drainway across Danby & Cadiz and neighboring playas. It is most surprising to find the fish in such isolate places, like Saratoga Springs or Devil's Hole, in Death Valley NP, or Quitobaquito Springs in Organ Pipe NP, sitting basically on the international border, south of Ajo.

Sand dune fields. The West wouldn't be the same without its sand dune fields. It is slightly curious how winds can sort out fine sand from soils and blow it into big piles of loose material. Pick up some dune sand and feel its very fine uniform nature--wind can only blow small particles around. Sand dunes are always located at some kind of 'null point' in a valley, some place where winds blow equally from all directions, or else the wind encounters cliffs and looses its speed, as along some coastal regions of northern California and Oregon where the onshore winds encounter high hills. The largest sand dune field in the Southwest is along coastal Sonora and extending into the Salton Valley of southern California. These dunes are called the Gran Desierto in Mexico, and the Algodones dunes (~cottonball dunes) between Yuma and El Centro. The Mohawk dunes in Mohawk Valley (east of Yuma) (Figure 53) are visible from I-8. Very tall dunes are found near Stovepipe Wells in Death Valley (Figure 55) where afternoon temperatures on a June afternoon can exceed 150 deg F when measured in open shade 18 inches above the sand - while several kinds of plants like mesquites and creosotes prosper because the sand is like a sponge, retaining moisture from rains two-three feet down below the heat. Dig a trench with your hands and find out. The tallest dunes of the region may be within Great Sand Dunes National Monument in the San Luis Valley of Colorado (~1,200 ft tall) ) (Figure 54). Satellite photos now tell us that winds across the entire Pacific Ocean from the west bring quartz and clay dust from the Gobi Desert to the Southwest, and that's the source of some of the clay in desert soils, and some guess this may be a third of the total clay in our soils. The book by Holmes tells it all.

Basically all sand dunes are composed of fine quartz sand with little other mineral present. A grand exception to this is White Sands National Monument east of Alamogordo New Mexico (Figure 56) where the sand is extra bright white and composed of granules of gypsum (calcium sulfate). There is a lesson here about big cycles over long periods on Earth. How did the gypsum sand get there?

When seawater evaporates, it produces first a thinner layer of gypsum, and then, if the process continues to complete evaporation, a thicker bed of salt (sodium chloride) will deposit. The floor of the Mediterranean is covered with gypsum and salt beds over a hundred feet thick which were formed when the sea was cut off from the Atlantic about 5 m.y. ago and evaporated to total dryness. Then the dam at Gibraltar broke and the sea re-flooded.

In late Paleozoic time (Permian period) the American West was covered by occasional lagoons of the paleo-Pacific ocean that dried out and left behind beds of gypsum. These beds can be seen in a remote region of the Arizona strip country (north of the Grand Canyon) and south of St. George Utah. The topmost beds in the Sacramento Mts, just east of White Sands New Mexico are Permian limestones that contain a thick gypsum bed (visible in a roadcut just before entering Cloudcroft from the west). This water-soluble gypsum washed down streams into the Lucero Valley's ice age playa lake called Lake Lucero. Then seasonal high winds have come along and eroded the gypsum beds of the playa into granules that have gathered east of the dry lake into a very imposing White Sands gypsum dune field. Eventually the mountains will go away and the gypsum will be free to travel down some new river, not yet conceived, back into the sea, where it originated. Length of the cycle?--several hundreds of millions of years.

Section 29. Temperature and climate history

Index of Pages

CPNHA Geology Home Page