Читать книгу Backpacking Arizona. From Deep Canyons to Sky Islands онлайн

54 страница из 56

Granite Gorge from Tonto Trail

Cliff and Terrace

In the canyon, the horizontal, sedimentary rock layers erode according to their hardness. Hard rocks such as limestone and sandstone form cliffs, while soft rocks such as shale and siltstone form slopes. Since these types of rocks tend to alternate in the cross section, the Grand Canyon has a staircase appearance of alternating cliffs and slopes. Terraces form where the soft layers are especially thick, so that the cliffs above retreat back and expose large, relatively level expanses of the lower cliff-forming layer. In the case of the Esplanade terrace along the upper South Bass Trail eastward, the surface of the terrace is formed on the upper sandstone member of the Supai formation (sometimes called the Esplanade sandstone), which tends to form a cliff about 200 feet high. In the central Grand Canyon, from the South Bass Trail westward, the Hermit shale thickens and causes the overlying Coconino sandstone to retreat back from the Esplanade Rim. Such terraces are vital for foot travel through the canyon. West of this point, the Esplanade forms the major route of travel. To the east, the Esplanade fades away because the Hermit shale is thinner. At the same time, the Bright Angel shale becomes thicker, forming a new terrace, the Tonto Plateau, on top of the resistant Tapeats sandstone. As you hike along the ridge at the head of Bass Canyon, you can look out at the Esplanade to the west, and down Bass Canyon at the Tonto Plateau to the north.