Grand Canyon

3x puncte

categorie: Engleza

nota: 9.38

nivel: Liceu

Briefly summarized, the geologic history of the canyon strata is as follows. The crystallized, twisted, and contorted unstratified rocks of the inner gorge at the bottom of the canyon are granite and schist about two billion years old. Overlying these very ancient rocks is a layer of limestones, sandstones, and shales that are more than 500 million years old.

On top of these are ro[...]
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Briefly summarized, the geologic history of the canyon strata is as follows. The crystallized, twisted, and contorted unstratified rocks of the inner gorge at the bottom of the canyon are granite and schist about two billion years old. Overlying these very ancient rocks is a layer of limestones, sandstones, and shales that are more than 500 million years old.

On top of these are rock strata composed of more limestones, freshwater shales, and cemented sandstones that form much of the canyon's walls and represent a depositional period stretching over 300 million years. Overlying these canyon rocks is a thick sequence of Mesozoic Era rocks (245 to 66.4 million years old) that form precipitous butte remnants and the vermilion, white, and pink cliff terraces of southern Utah but which have been entirely eroded away in the area of the Grand Canyon proper.

Of relatively recent origin are overlying sheets of black lava and volcanic cones that occur a few miles southeast of the canyon and in the western Grand Canyon proper, some estimated to have been active within the past 1,000 years. The cutting of the mile-deep Grand Canyon by the Colorado River is an event of relatively recent geologic history that began not more than six million years ago, when the river began following its present course. The Colorado River's rapid velocity and large volume and the great amounts of mud, sand, and gravel it carries swiftly downstream account for the incredible cutting capacity of the river.

Prior to the building of the Glen Canyon Dam, the sediments carried by the Colorado River weremeasured at an average of 500,000 tons per day. Conditions favourable to vigorous erosion were brought about by the uplift of the region, which steepened the river's path and allowed deep entrenchment. The depth of the Grand Canyon is due to the cutting action of the river, but its great width is explained by rain, wind, temperature, and chemical erosion, helped by the rapid wear of soft rocks, all of which steadily widened it.

Amazingly, the canyon was cut by a reverse process, for the river remained in place and cut through the rocks as the land moved slowly upward against it. Only thus can be explained the canyon's east-to-west course across a south-facing slope and the presence of plateaus that stand across the river's course without having deflected it. The most significant aspect of the environment that is responsible for the canyon is frequently overlooked or not recognized.

Were it not for the arid climate in the surrounding area, there would be no Grand Canyon. Slope wash from rainfall would have removed the canyon walls, the stairstep topography would long ago have been excavated, the distinctive sculpturing and the multicoloured rock structures could not exist, the Painted Desert would be gone, and the picturesque Monument Valley would have only a few rounded hillocks.

Biological past and present

Plant and animal fossils are not abundant in the Grand Canyon's sedimentary rocks and are confined mostly to primitive algae and mollusks, corals, trilobites, and other invertebrates. Animal life in the Grand Canyon area today is varied and abundant, however. The common animals are the many varieties of squirrels, coyotes, foxes, deer, badgers, bobcats, rabbits, chipmunks, and kangaroo rats. Plant life is also varied. In the bottom of the canyons are willows and cottonwoods, which require abundant water during the growing season.

At the other end of the moisture scale are drought-resistant plants such as the yucca, agave, and numerous species of cactus.
On the canyon rims, north and south, there is a wide assortment of plant life. Typical of the South Rim is a well-developed ponderosa pine forest, with scattered stands of piñon pine and juniper. Bush vegetation consists mainly of scrub oak, mountain mahogany, and large sagebrush. On the North Rim are magnificent forest communities of ponderosa pine, white and Douglas fir, blue spruce, and aspen. Under less optimum conditions the plant life reverts to the desert varieties.
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