Some of the most effective means of erosion at Mount Rainier are landslides and mudflows. Erosion of this kind is sometimes spectacular. Within an interval of only minutes or a few hours huge masses of rock may fall, slide, or flow off the volcano and move far downvalley. Large landslides have occurred at many other places in the park—one in the area northeast of Mount Rainier is so conspicuous that its source has been named Slide Mountain. The ragged scar left by another slide near Grand Park is aptly called Scarface. You cross a slide on the Mather Memorial Parkway (U.S. Highway 410) just north of Cayuse Pass. Broken and jumbled rock debris of many sizes borders both sides of the highway there. This landslide broke loose in rocks of the Ohanapecosh Formation, slid downslope to the bottom of the valley, and dammed Klickitat Creek to form Ghost Lake. Rocks have also slid downslope on the west side of Backbone Ridge and on the east side of the Ohanapecosh River valley a short distance north of Ohanapecosh campground. The slide on Backbone Ridge is still slowly moving today. Another slide moves a few inches each year on the west side of the Nisqually River valley about 1 mile northwest of the visitor center at Paradise Park. You can recognize the slide by a jagged horizontal crack 1,000 feet long at its top. A far more spectacular variety of landslide occurs when a mass of rock drops from a cliff to form a rockfall. The largest rockfalls on Mount Rainier in historic time occurred in December 1963 on the east flank. Masses of rock hundreds of feet across fell repeatedly from the steep north face of Little Tahoma Peak onto Emmons Glacier. The rock masses shattered into dust and countless fragments, fanned out across the glacier, then avalanched down the steep ice surface at tremendous speed. When the avalanches reached the end of the glacier they shot out into space as sheets of rock debris. As these hurtling sheets settled toward the valley floor, a cushion of compressed air formed beneath them, comparable to the air cushion that momentarily buoys up a sheet of plywood that is dropped onto a flat surface. Air that was trapped beneath these speeding avalanches reduced friction and permitted one of the avalanches to move almost 2 miles beyond the end of the glacier. This avalanche passed completely over a small wooden gage house about 5 feet high on the valley floor without damaging it, then ran headlong into the north base of Goat Island Mountain where it scraped away trees and bushes. A later avalanche stopped just short of the gage house, and wind that was expelled from beneath the rock debris blasted the still-undamaged house several hundred feet forward. It now rests in the scar left by the earlier avalanche on the side of Goat Island Mountain.