...n about the canyon. By the 1870s, folloing the exploration of John esley Poell and others, extensive reports on the geography, geology, botany, and ethnology of the area ere being published.Grand Canyon National Park, no containing 1,904 square miles 4,931 square km, as created in 1919. Its area as greatly enlarged in 1975 by the addition of the former Grand Canyon National Monument and Marble Canyon National Monument and by portions of Glen Canyon National Recreation Area, as ell as other adjoining lands. The North and South rims are connected by a 215-mile- 346-kilometre- long paved road and by a transcanyon trail. Scenic drives and trails lead to all important features. Mule-pack trips don the canyon and rides don the river in rafts and poer-driven craft are intensively sought-after ays of vieing and experiencing the vast beauty of the canyon. Many pueblo and cliff-deller ruins, ith accompanying artifacts, indicate prehistoric occupation. There are five Indian tribes living on nearby reservations.Geologic historyAlthough its aesome grandeur and beauty are the major attractions of the Grand Canyon, perhaps its most vital and valuable aspect lies in the time scale of Earth history that is revealed in the exposed rocks of the canyon alls. No other place on Earth compares ith the Grand Canyon for its extensive and profound record of geologic events. The canyons record, hoever, is far from continuous and complete. There are immense time gaps many millions of years are unaccounted for by gaps in the strata in hich either vast quantities of Earth materials ere removed by erosion or there as little or no deposition of Earth materials. Thus rock formations of vastly different ages are separated only by a thin, distinct surface that reveals the vast unconformity in time.Briefly summarized, the geologic history of the canyon strata is as follos. The crystallized, tisted, and contorted unstratified rocks of the inner gorge at the bottom of the canyon are granite and schist about to 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, freshater shales, and cemented sandstones that form much of the canyons alls 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, hite, and pink cliff terraces of southern Utah but hich have been entirely eroded aay in the area of the Grand Canyon proper. Of relatively recent origin are overlying sheets of black lava and volcanic cones that occur a fe miles southeast of the canyon and in the estern Grand Canyon proper, some estimated to have been active ithin 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, hen the river began folloing its present course. The Colorado Rivers rapid velocity and large volume and the great amounts of mud, sand, and gravel it carries siftly donstream 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 eremeasured at an average of 500,000 tons per day. Conditions favourable to vigorous erosion ere brought about by the uplift of the region, hich steepened the rivers path and alloed deep entrenchment. The depth of the Grand Canyon is due to the cutting action of the river, but its great idth is explained by rain, ind, temperature, and chemical erosion, helped by the rapid ear of soft rocks, all of hich steadily idened it. Amazingly, the canyon as cut by a reverse process, for the river remained in place and cut through the rocks as the land moved sloly upard against it. Only thus can be explained the canyons east-to-est course across a south-facing slope and the presence of plateaus that stand across the rivers course ithout having deflected it.The most significant aspect of the environment that is responsible for the canyon is frequently overlooked or not recognized. ere it not for the arid climate in the surrounding area, there ould be no Grand Canyon. Slope ash from rainfall ould have removed the canyon alls, the stairstep topography ould long ago have been excavated, the distinctive sculpturing and the multicoloured rock structures could not exist, the Painted Desert ould be gone, and the picturesque Monument Valley ould have only a fe rounded hillocks.Biological past and presentPlant and animal fossils are not abundant in the Grand Canyons 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, hoever. 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 illos and cottonoods, hich require abundant ater during the groing 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 ide assortment of plant life. Typical of the South Rim is a ell-developed ponderosa pine forest, ith scattered stands of pion 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, hite and Douglas fir, blue spruce, and aspen. Under less optimum conditions the plant life reverts to the desert varieties.Grand Canyon SeriesMajor division of rocks in northern Arizona dating from Precambrian time about 3.8 billion to 540 million years ago. The rocks of the Grand Canyon Series consist of about 3,400 m about 10,600 feet of quartz sandstones, shales, and thick sequences of carbonate rocks. Spectacular exposures of these rocks occur in the Grand Canyon of the Colorado River in northestern Arizona, here they overlie the strongly deformed and contorted Vishnu Schist, the angularity of hich stands in bold contrast to the almost horizontal bedding of the Grand Canyon Series. The Grand Canyon Series actually dips slightly eastard and is separated from the overlying Cambrian sandstones by a major erosion surface unconformity. A conglomerate as deposited on the eroded surface of the Vishnu Schist. Limestones, shales, and sandstones occur over the conglomerate and are thought to represent shallo ater deposits. The area of deposition as probably a large deltaic region that as sloly subsiding, alloing great thicknesses of sediment to accumulate near sea level . The presence of Precambrian organisms is indicated by calcareous algaelike structures in the carbonate rocks, as ell as by tracks and trails of orml...
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