对美国大峡谷的个人感受
第一次去大峡谷之前看了不少资料，知道这条著名的峡谷由此刻安静流淌在谷底的科罗拉多河用600万年的光阴一点点切割而成，峡谷长达446公里，最宽处近30公里，最深处垂直落差近2公里。
河水层层切割出的岩石断面展现了从20亿年前至今的地球形成史。
这些数字，如果用具象些的概念来对照，能让人对大峡谷的壮观有个大致的想像。
但真正的感受，是数字无法衡量的，只有你亲身站在绝壁边，放眼眼前的旷世奇景，才能从心底里发出对造物的由衷赞叹。
按照事先的路线，傍晚时分，我们开车经横贯美国的66号公路来到一处叫威廉姆斯的小镇。
典型的美国西部小镇，高低错落的小楼排满道路两侧。
我们下榻的旅馆叫大峡谷铁路酒店，两层楼的酒店有著装饰得非常西部的宽敞大厅和深邃得近乎不可思议的长廊。
早晨起床，发现房间不远处另有扇通向外面的门，推门出去，眼前出现的是几条铮亮的铁路：这外形狭长的酒店依著铁轨而建，而我们今天早晨就会在这里乘上火车前往大峡谷。
走下峡谷的那天早晨，天空起了阴云。
大峡谷垂直落差达2公里，越往深走气温越高，谷顶阳光明媚时，谷底气温超越人体体温属於家常便饭，多云天对穿越者无疑是一个福音。
去谷底只有两个办法，走路或者骑骡子。
骑骡入谷，是大峡谷上百年来长盛不衰、最具风景的一项活动。
不过，在骡背上摇晃著，随著骡队前进，使得拍照片成为泡影。
换上登山靴，将水瓶塞进行囊，步行才是惟一的可能。
从大峡谷的南坡下行，有3条羊肠小道可供选择。
最热闹的是在旅馆区的光明天使小径（Bright Angel Trail)，从早到晚熙熙攘攘。
所有来大峡谷的人都会从这里往下走上几步，感觉一下站在峡谷内部的心情。
隐士小径（Hermit Trail)名副其实，这条在70年前就被探险者们放弃的路上风光壮丽，但没有任何可靠水源，只有少雪N志和体力都坚强无比的勇士才会背足食品和水走过。
我们选择的南凯坝小径（South Kaibab Trail）难度介於两者之间，从旅馆需要乘车才能到达悬崖边的起点，很少有游客专程来这里，峡谷小径上自然也多了几分宁静。
出发时天有些阴，走上小径，没多远路面就变得坡度陡峭，每向前踏出一步，都会是回程时气喘吁吁的攀登。
第一程的路最为险峻，多数是在山崖上硬用炸药炸出的小径，盘旋曲折。
到达平衡些的地方回头看，刚才走过的路好像一丝细线嵌在直上直下的山崖之上，线上那些缓慢移动的小点就是刚才走过的我们。
不知不觉间山谷里飘起了丝丝小雨。
走过一处转角，眼前豁然开朗，山势壮丽，谷中云雾蒸腾，同行的朋友们不由自主地发出了赞叹声。
带队的当地朋友笑了，因为路过此地的人都会“唔哇”地感叹，所以就叫做“唔哇点”(Ooh-aah Point)。
迎面来了一个骡队，骑在骡上的游客看似威风，脸上多少还是有些紧张。
因为骡队必须和步行者共享同一条山路，所有的骡子都被训练得贴著悬崖边走，让步行者走在靠近崖壁的一侧。
这样的好处是，万一骡子发飙，不至於将行人直接挤下山去。
四条腿的骡子经过每天上下峡谷绝壁的训练，比我们这些偶尔为之的行人走得稳健得多。
骑骡子进峡谷有个好玩且实用的规矩。
预订的客人经常会在最后一秒钟被道路的险峻吓怕，到了悬崖边，带队的会最后问一下各位是否准备完毕。
只要骡子还没有走下山道，还可以要求全价退票。
但一旦进入峡谷，哪怕才几步路再改主意，尽可以下骡子走人，钱是不会退的。
美国的大峡谷多深?
美国西北部的大峡谷。
由水深切而成。
自北部州界之马布尔峡谷至界附近格兰德瓦什崖，全长约350公里，深1，800米；谷顶部宽8-25公里，谷底水面线的宽度小于1公里，个别地方仅120米。
两侧谷壁呈阶梯状。
气候干燥，植物稀少，谷壁可观察到从古老的元生代到新生代的各期地层，在阳光下显示出各种不同色彩。
其中最深的一段峡谷（长约170公里）辟为国家公园。
位于美国西北部中游、科罗拉多高原的西南部，是地球上最为壮丽的景色之一，有人说，在太空唯一可用肉眼看到的就是。
是北美洲主要河流。
它从发源流向西南，进入墨西哥西北部。
全长2330公里。
流经怀俄明、科罗拉多、犹他、、内华达、亚利桑那和七个州，在科罗拉多高原上共切割出19条主要峡谷，其中最深、最宽、最长的一个就是。
该峡谷起于马布尔峡谷，终端为格兰德瓦什崖，全长446公里，是世界上最长的峡谷之一。
峡谷顶宽6至28公里，最深处1800米。
从谷顶到谷底需3至4小时。
谷底两岸的宽者小于1公里，窄处仅120米。
两侧的谷壁呈阶梯状。
谷底水面不足1000米宽，夏季冰雪融水下注，水深增至18米。
山石多为红色。
从谷底至顶部沿壁露出从到新生代各期的系列岩系，水平层次清晰，岩层色调各异，并含有各地质时期代表性的生物化石，故有“活的地质史教科书”之称。
1919年通过法案，将其中长约170公里、面积2728平方公里、最深的一段峡谷辟为国家公园。
现在，每年都有很多国内外游客到这里旅游，欣赏大自然的鬼斧神功，认识地球亿万年的神奇变化。
这里还是拍摄美国西部片的好场地，那荒野而神秘的景色，相信你早已熟悉不过了。
以下我们要向你介绍的是大峡谷的成因和它那极富魅力景色特写，那些美丽的图片一定会令你更加向往。
科罗拉多高原为典型的“桌状高地”，也称“桌子山”，即顶部平坦侧面陡峭的山。
这种地形是由于侵蚀（下切和剥离）作用形成的。
在侵蚀期间，高原中比较坚硬的岩层构成河谷之间地区的保护帽，而河谷里侵蚀作用活跃。
这种结果就造成了平台型大山或堡垒状小山。
科罗拉多高原是北美古陆台伸入科迪勒拉区的稳定地块，由于相对稳定，地表起伏变化极小，而且在结晶岩的基底上覆盖了厚厚的各地质时期的沉积，其水平层次清晰，岩层色调各异，并含有各地质时期代表性的生物化石。
岩性、颜色不同的岩石层，被外力作用雕琢成千姿百态的奇峰异石和峭壁石柱。
伴随着天气变化，水光山色变幻多端，天然奇景蔚为壮观。
峡谷两壁及谷底气候、景观有很大不同，南壁干暖，植物稀少；北壁高于南壁，气候寒湿，林木苍翠；谷底则干热，呈一派荒漠景观。
蜿蜒于谷底的科罗拉多河曲折幽深，整个大峡谷地段的河床比降为每千米150厘米，是密西西比河的25倍。
其中百分之50的比降还很集中，这就造成了峡谷中部分地段河水激流奔腾的景观。
因为如此，沿峡谷航行漂流成为引人入胜的探险活动。
参考资料：
美国大峡谷的形成原因是什么
How was it formed?The truth is that no one knows for sure though there are some pretty good guesses. The chances are that a number of processes combined to create the views that you see in todays Grand Canyon. The most powerful force to have an impact on the Grand Canyon is erosion, primarily by water (and ice) and second by wind. Other forces that contributed to the Canyon's formation are the course of the Colorado River itself, vulcanism, continental drift and slight variations in the earths orbit which in turn causes variations in seasons and climate. Water seems to have had the most impact basically because our planet has lots of it and it is always on the move. Many people cannot understand how water can have such a profound impact considering that the Canyon is basically located in a desert. This is one of the biggest reasons that water has such a big impact here. Because the soil in the Grand Canyon is baked by the sun it tends to become very hard and cannot absorb water when the rains to come. When it does rain the water tends to come down in torrents which only adds to the problem. The plants that grow in the Grand Canyon tend to have very shallow root systems so that they can grab as much water as possible on those rare occasions when it does rain. Unfortunately these root systems do nothing to deter erosion by holding the soil in place. Now you've got lots of water, no place for it to go, but down to the Colorado River, and nothing holding the soil and rock in place. The result is frequently a flash flood roaring down a side canyon that can move boulders the size of automobiles, buses and even small houses. If automobiles, buses and small houses are in the way then it will take them too. Luckily no one builds houses in the Grand Canyon so that's not a problem but there are a few autos, vans and buses sitting at the bottom of the Colorado. This mass that moves down a side canyon during a flash flood is more like a fast flowing concrete than water and it can be very dangerous. You should always be well informed of weather conditions when you are hiking through side canyons in the Grand Canyon. After erosion by liquid water the next most powerful force is probably its solid form, ice. In the colder months, especially on the north rim, water seeps into cracks between the rocks. These cracks can be caused by seismic activity, or by the constant soaking and drying of the rocks. When the water freezes it expands and pushes the rocks apart and widens the cracks. Eventually rocks near the rim are pushed off the edge and fall into the side canyons. These rocks sometimes hit other rocks and are stopped but on occasion one fall by a large rock will cause a cascading effect and create a rock fall that will alter the landscape drastically in the side canyon. Debris from rock falls piles up at the bottom of the side canyons and is then carried down to the Colorado River the next time there is a flash flood. Rock falls frequently take out sections of trail in the Grand Canyon requiring the Park Service to close these trails until they can be repaired. Once the ice had pushed the rocks off the edge and the water in the flash floods has carried them down to the river, then the Colorado itself takes over. The erosive action of the Colorado has been severely constrained by the building of the Glen Canyon Dam, which ended the annual spring floods, but there is still a lot of water flowing relatively quickly through a very narrow gorge. Before building the dam the Colorado River had spring floods that would exceed a flow rate of 100,000 CFS. All of that snow melting in the Colorado Rockies came pouring down through the Grand Canyon in May and June, every year, like clock-work. These spring floods were considerably larger than todays trickle of 8,000-10,000 CFS at low water and even the 20,000 CFS peak flow rates. The Colorado's spring floods used to carry away all of the debris that was deposited in the main channel by the flash floods, but todays mediocre flow rates have a tough time doing the job. It still gets done to some extent, it just takes a lot longer. In the process of moving the rocks and sediment down the river to the Pacific Ocean the bed of the river is scoured by all of this fast moving debris which slowly eats away at the banks and bed of the river. This causes the river to widen and cut down deeper into the lower rock layers. Another cause for the slowing of the erosive force of the Colorado River is the fact that it is now trying to cut through harder granites and schists found at the bottom of the Canyon instead of the softer limestones, sandstones and shales near the top. This rock takes a lot longer to erode and a slower moving river means it takes even longer. --------------------------------------------------------------------------------Where did all of the rock come from?Geologists have this question pretty much wrapped up, aside from some missing layers, or unconformities, that have been completely eroded away. Again there were a number of forces at work and this is where continental drift, vulcanism and climatic change come into play. The fact that the Earth's continents are not fixed in place but rather float on a sea of molten rock, means that they move around quite a bit, relatively speaking. The surface of the Earth is composed of about twenty of these plates which form its crust. Seven of these plates are very large and consist of entire continents or sea floors and the rest are smaller in comparison. The plates are average out to be about 50 miles or 80 kilometers thick and float on top of the Earth's mantle. The plate which contains the Grand Canyon, the North American plate, was at one time considerably further south than its present location and therefore had a much different climate. In time it has gradually moved north and rotated about ninety degrees to its present location and configuration. The continents in motion, the red dot indicates the approximate location of the Grand Canyon region. Click here or on the image above for more information on the continental drift theory. Click here to visit the USGS site This Dynamic Earth: the Story of Plate Tectonics The North American Plate is moving west and is colliding the Pacific Plate which is moving towards the northwest. The Pacific Plate is also expanding from its middle and its eastern edge is being subducted beneath the North American Plate as it comes into contact with it. Oceanic plates are typically subducted beneath continental plates because they area heavier. As pressure increases while they are being subducted they tend to get heavier still and to some extent they start to fall and pull more plate along with them. As the Pacific Plate moves beneath the North American Plate the rock of which it is composed is superheated and water is released and begins to rise. This water, which is extremely hot, causes lighter minerals to melt and forms lava which feeds the chain of volcanoes on the eastern edge of the Pacific Rim which runs from Alaska to Chile. The conflict between the plates is also frequently responsible for mountain building activity. As the plates are forced together they sometimes buckle which causes mountain ranges to be formed along the contact point. This is how the Rocky Mountains, the Sierra Nevada and the costal mountains of California were formed and how the Aleutian Island are being formed today. A much older range of mountains, which geologists suspect were much higher than todays Rocky Mountains and may even have rivaled the Himalayas, now forms the base of the Grand Canyon. The rocks that made up these mountains are about 1.7 billion years old, or about one-third the age of our planet. These mountains have long since eroded away and sedimentary deposits have covered them over. The sediments that covered the roots of these ancient mountains were deposited by a series of advancing and retreating ocean coast lines. As the climate of our planet warms and cools the median sea level of the planet rises and falls due to the melting and freezing of the polar caps. When the sea level rises, land areas which are close to the coast and relatively low in altitude are sometimes submerged. This was the case with the land area of the Grand Canyon and is why so many different sedimentary rock layers exist. Each of these was formed by a different period in which the ocean moved in and covered the land, stayed for a while, and then retreated again. Limestone deposits are created when the ocean moves in and slates, shales and mudstone deposits are created when the ocean moves out and the area is covered by silts washing into the retreating ocean. How do we know this? Well, the fact is that most of the rock in the Grand Canyon is composed of sedimentary rock which can only be formed at the bottom of the ocean or in shallow coastal plains. The Kaibab Limestone which is the current top of the Grand Canyon is composed mostly of a sandy limestone, with some sandstone and shale thrown in for good measure. This means that it was probably formed in a shallow sea near the coast. The fact that it contains fossils of creatures that used to live in the ocean, like brachiopods, coral, mollusks, sea lilies, worms and fish teeth, only tends to reinforce this belief. The intrusion of sandstone and shales into this later means that at times the layer was also above the surface of the water but still very close to the edge. Sandstones are solidified sand which are typically fields of sand dunes or beaches, and shales are solidified mud which are common to river deltas. By dating the fossils found in the rock of the Kaibab Limestone, geologists have determined that it is approximately 250 million years old, and this is the youngest layer. So where are the younger rocks? The younger rocks have already been eroded away by the forces of nature, at least in the immediate vicinity of the Grand Canyon. Some of the younger layers, like the Navajo Sandstone of which the Vermilion Cliffs and the rock of Zion National Park are composed, can be found in the region north of the Grand Canyon. Going even further north results in even younger rocks as can be seen in Bryce Canyon. The area from Bryce Canyon down to Grand Canyon is typically referred to as the Grand Staircase. Cross sectional view of the Colorado Plateau showing the Grand Staircase --------------------------------------------------------------------------------Why does it look like it does?The reason that it looks the way does is due to the sequence in which the events that help to create it happened. We already know that there was once a very tall chain of mountains in the area that occupied the Grand Canyon. These mountains were, over many millions of years, eventually eroded away to form a level plain. Fluctuations in climate then caused the oceans to move in over successive periods and each time a new rock layer was deposited. The rock layers were deposited one on top of the other and sometimes there were long periods in between in which some of the upper layers were eroded away, sometimes completely. And now the Colorado River comes into play. The ancestral Colorado River came into being when the Rocky Mountains to the east of the Grand Canyon were formed, at sometime around 60-70 million years ago, as the primary western drainage for these mountains. Over millions of years the course of this ancestral river changed its course a number of times as the terrain around it was altered. The course of the ancestral Colorado River probably started in Colorado and at one point it entered the region of Marble Canyon, but that is about all that can be agreed upon at this point. Some geologists believe that very young rock layers to the west of the Grand Canyon, dated at only 5 and 10 million years old, and through which the Colorado now flows, indicate that the river could not have been flowing there prior to that time. The river had to cut through these layers after they were deposited. The search for another exit for the Colorado River from the Grand Canyon has been a hotly debated issue. Some geologists believe that it flowed out of Marble Canyon where the Little Colorado now enters, others believe that it exited near present day Diamond Creek and still others believe that it exited through massive caves in the Redwall Limestone. The most likely exit at this point seems to be up through Kanab Creek which would have had the ancestral river flowing back up into Utah and then across Nevada and California to the Pacific. At around 17 million years ago, while the river was flowing across this ancient landscape, the land mass know as todays Colorado Plateau began to uplift. The uplift was caused by pressures deep with the Earth and may have been caused by additional conflict between the North American Plates and the Pacific Plates. This process continued until around 5 million years ago which interestingly enough is the date of the sedimentary layers just west of the plateau. At its greatest hieght the Colorado Plateau was once about three miles above sea level. The rise of the plateau probably prevented the seas from submerging it again and instead the topmost layers were eroded away and carried into the sea. The most favorable currently accepted theory is that the Colorado River continued to cut through the Colorado Plateau while the land rose around it. At some point around 5 million years ago something happened to cause the Colorado to change its course and exit via its present route down to the Gulf of California. The most likely cause for the change in its course was probably due to it being captured by another river, which was draining the western portion of the Colorado Plateau. This other river eroded northward along the San Andreas fault, then eastward and eventually entered the Grand Canyon and joined with the Colorado near present day Kanab Creek. The Colorado would then have abruptly changed its course and flowed out this newly formed exit. Much of the eastern Grand Canyon was already formed by the time the river changed its course. Side canyons had formed along fault lines in the rock and these were eroded away and the rock within them carried down to the Colorado. The Colorado River took all of the rock that was put into it and carried it off to the Pacific Ocean. Over many more millions of years the erosion along the course of the Colorado continued to widen the Canyon to present the vistas that you see today. Before the Glen Canyon Dam was built the Colorado River used to carry three cubic miles of sediment into the Pacific Ocean every hundred years. --------------------------------------------------------------------------------When did all this happen?The Earth was formed approximately 5 billion years ago. The roots of the ancient mountain range that now lies at the bottom of the Grand Canyon were formed about 1.7 billion years ago. There is then an unconformity of about 450 million year in which the rocks are missing. At 1.25 billion years ago the first sedimentary layer, the Bass Formation, was laid down. Ancient coastal dwelling colonies of algae known as Stromatolites are preserved within this layer and indicate that the area was coastal at that time. At 1.2 billion years ago the sea retreated leaving mud flats behind which eventually became the Hakatai Shale. At 1.19 billion years a similar layer was deposited which is known as the Dox Formation. This was again formed of mudstones and shales and contains ripple marks as well as other features that indicate that it was close to the coast. Between 1.25 and 1.1 billion years ago there was also some volcanic activity with the region of the Grand Canyon and this is when the Cardenas Basalts were formed. Between 1 billion and 825 million years ago additional coastal and shallow sea formations, which are now classified as the Chuar group, were deposited. There is then another unconformity of about 250 million years in which new rock layers were probably laid down but were completely eroded away. The Tapeats Sandstone was then deposited around 550 million years ago along long vanished coastline. There are places in the Canyon in which in which off shore islands have been found imbedded within this layer. The Bright Angel Shale was deposited around 540 million years ago and indicates that the ocean was again advancing. The Muav Limestone was deposited around 530 million years ago at the bottom of a shallow sea. The thick layer of Redwall Limestone which began to deposited around 330 million years ago indicates that the land was submerged for a great deal of time. The Supai Group which rests atop the Redwall is dated at 300 million years ago and indicates that it was formed in an above water and coastal environment. The Hermit Shale which was deposited around 280 million years ago contains many plant fossils which indicate that it was also above water. The Coconino Sandstone represents the remains of a vast sea of sand dunes which was blown down from the north around 270 million years ago. The layers found within Toroweap Formation contains both sandstone and limestone, indicating that it was sometimes coastal and sometimes submerged. These layers date to around 260 million years. The top layer of the Grand Canyon, the Kaibab Limestone, contains many marine fossils which indicate that it originated at the bottom of the sea. This layer is around 250 million years old. Rock layers younger than 250 million years have been eroded away and no longer exist in the immediate vicinity of the Grand Canyon. The Rocky Mountains begin to form 60-70 million years ago and at some point later the Colorado River is born. At this point there are at least two popular theories which describe what happens next: Around 20 million years ago the Colorado River begins to carve into the Grand Canyon at its eastern end, Marble Canyon, and probably exiting via Kanab Canyon. At 17 million years ago the Colorado Plateau begins to uplift and causes the river to cut deeper. Around 5 million years ago the uplift ceases and another river working its way northward along the San Andreas fault and eastward along the western Colorado Plateau captures the Colorado River. OR Around 35 million years ago the Kaibab Plateau begins to uplift and diverts the ancestral Colorado, which was already established on a course very similar to that of today, to the southeast. The cut-off western portion, now named the Hualapai Drainage System, contines to drain the western region. About 12 million years ago the Colorado's path to the sea is blocked and a huge lake, Lake Bidahochi, is formed. Eventually the Hualapai cuts back through the southern portion of the plateau and recaptures the Colorado. Lake Bidahochi is drained and becomes the Little Colorado River.
