[by:www.51voa.com] [00:00.00]更多听力请访问51VOA.COM [00:09.00]Our national parks journey this week takes us to the western state of Colorado. [00:18.84]There, you will find a 77-kilometer-long canyon. [00:25.68]Its exposed rocks are among the oldest you will find on Earth. [00:32.36]The canyon is so narrow and so deep that sunlight shines only briefly at the bottom at midday. [00:45.88]That frequent darkness is what has given this canyon its name. [00:53.16]Welcome to Black Canyon of the Gunnison National Park! [01:07.56]Black Canyon seems impossibly steep and narrow. [01:13.48]The fast moving Gunnison River formed the canyon over millions of years. [01:22.32]As the river flows through the canyon, [01:26.00]it drops an average of 18 meters per kilometer. [01:33.04]The Gunnison River loses more elevation in its 77 kilometer canyon stretch [01:41.84]than the Mississippi River does in 2,400 kilometers from Minnesota to the Gulf of Mexico. [01:53.52]This loss in elevation causes the river to gain speed and force. [02:01.24]Over time, that force has carved and cut the hard rock, [02:09.04]and created the wild and jagged rock formations found at Black Canyon. [02:17.56]That process is known as erosion. [02:21.92]At its widest, Black Canyon is 14 kilometers. [02:29.08]At its deepest point, it is 829 meters (kilometers×) from the canyon edge to the floor. [02:40.88]Black Canyon attracted little attention from early Western explorers. [02:48.84]In fact, no early Spanish explorers to the southwestern United States reported seeing Black Canyon. [02:59.36]The first written record about the canyon came in 1873. [03:07.40]The Ute Indians certainly knew of Black Canyon. [03:12.36]But only its rim, never the gorge, shows evidence of past human occupation. [03:21.84]Even today, the depth and steepness of Black Canyon [03:27.76]make it extremely difficult for humans to access. [03:34.28]Birds, however, have no problem surviving in the canyon environment. [03:42.40]Black Canyon is home to many kinds of birds. [03:46.92]They live at the canyon rim, along its walls, and by the river. [03:54.24]Mountain bluebirds and great horned owls live in the rim area. [04:01.56]The owls eat seeds and berries from the pinyon, juniper and oak trees found there. [04:11.04]Canyon wrens sing loudly within Black Canyon. [04:16.00]Visitors are more likely to hear the birds than seen them. [04:22.60]They build their nests on flat surfaces of the canyon walls. [04:28.48]So do peregrine falcons. They prey on other birds. [04:35.60]They dive at speeds up to 300 kilometers per hour to catch them. [04:43.64]American dippers build their nests in the gorge, next to the Gunnison River. [04:52.16]A gorge is a narrow valley between hills or mountains. [04:57.60]These dippers feed on insects and small fish. [05:03.36]Some people come to Black Canyon of the Gunnison National Park for world-class birdwatching. [05:13.20]But, it is the canyon itself that appeals to most visitors. [05:20.16]Walking along the rim, seeing the canyon's towering cliffs, [05:26.76]and hearing the powerful flow of the Gunnison River far below is an overwhelming experience. [05:37.56]At Black Canyon, visitors can view some of the planet's oldest exposed rocks. [05:46.68]They are Precambrian rocks. [05:50.32]Precambrian accounts for the Earth's history [05:54.92]from its very beginning up until about 540 million years ago. [06:02.72]Precambrian rocks are described as "basement rocks" [06:08.12]because they are usually buried deep under the surface. [06:14.76]The only become exposed when younger rocks have been cut away by erosion or other forces. [06:25.32]At Black Canyon, the Gunnison has eroded the canyon walls so greatly [06:32.32]that Precambrian rocks have become visible. [06:37.36]Often, the canyon's rock walls look like works of art. [06:42.80]Different kinds of rocks fold together and form colorful patterns. [06:50.00]The patterns change as the sun hits different parts of the walls. [06:57.28]One area of the canyon is known as Painted Wall. [07:02.00]It is the highest cliff in Colorado. [07:07.32]Long pink bands of rock cover this section of the canyon, [07:13.80]as if a person took a paintbrush to the wall. [07:18.72]This pattern formed as hot liquid from below the Earth's surface [07:26.00]forced its way into rock cracks. [07:29.92]As it slowly cooled, large crystals formed. [07:35.80]The bands get their pink color from a crystallized mineral called potassium feldspar. [07:46.44]Local people knew how just special Black Canyon was. [07:52.48]They began urging Congress to protect the canyon in the 1930s. [08:00.12]It became a national monument in 1933. [08:06.56]And in 1999 it became a national park. [08:12.88]The park protects more than 22 kilometers of the canyon. [08:18.84]Most visitors spend their time exploring the canyon rim by foot or by car. [08:28.16]Several kilometers of trails lead to many overlook points. [08:34.44]A road also leads to several breathtaking views. [08:40.56]The U.S. Congress named the lands below the canyon rim a Wilderness Preservation System. [08:50.00]This is meant to "protect forever the land's natural conditions." [08:56.65]Some adventurous travelers choose to explore this wilderness [09:03.64]by hiking down to the inner canyon. [09:08.16]There are no marked trails to guide them. [09:12.24]Hikers must first get a wilderness use permit. [09:17.28]These permits are free, but only a limited number are available each day. [09:24.88]Many of the paths are difficult to follow. [09:30.24]Some are also dangerous. [09:33.72]The Warner Point trail, for example, drops more than 820 meters. [09:41.64]Walking down takes more than two hours. [09:46.32]Walking back up can take up to four hours. [09:51.28]The park service warns that poison ivy grows wildly in the inner canyon. [09:59.08]There is also the risk of falling rocks. [10:03.64]But, to reach a place where no humans ever tried to live is the reward. [10:12.08]In the words of a 1901 explorer to Black Canyon's river area, [10:19.80]"Our surroundings were of the wildest possible description. [10:25.56]The roar of the water...was constantly in our ears... [10:30.68]Occasionally a rock would fall from one side or the other, [10:37.00]with a roar and crash, exploding like a ton of dynamite when it struck bottom, [10:45.60]making us think our last day had come." [10:56.20]I'm Ashley Thompson. [10:57.76]And I'm Caty Weaver. [10:59.80]更多听力请访问51VOA.COM