While hiking, you soon become aware that there is a large amount of pumice along the trails in Mount Rainier National Park. Pumice is a lightweight volcanic rock so full of air spaces that it will float on water. The air spaces, or bubbles, originated when fragments of gas-rich lava were explosively thrown into the air above the volcano, and the molten rock hardened before the gas could escape. If you examine pumice deposits in a trail cut, in a streambank, or in the roots of blown-over trees, you may also note that there is more than one layer (fig. 7). If you circle the volcano on the Wonderland Trail, you may notice that the greatest number of pumice layers are on the east side of the park, but the thickest single layer is on the west side. The explanation lies partly in the source of the pumice deposits, because some pumice was erupted not by Mount Rainier but by other volcanoes in the Cascade Range of Washington and Oregon and brought to the park by strong southerly or southwesterly winds. The layers of pumice thrown out by Mount Rainier within the last 10,000 years lie mostly on the east side of the volcano. Strong winds evidently swept eruption clouds to the east during the outbursts and prevented the pumice from falling west of the volcano. This pattern of distribution, coupled with the coarsening and thickening of the pumice toward the volcano, reveals that the layers were erupted by Mount Rainier. [Illustration: An old lava flow from Mount Rainier which forms Rampart Ridge west of the meadow at Longmire. The thick lava flowed down an old valley floor and cooled and solidified. Rivers then eroded new valleys along both sides of the flow. These new valleys, subsequently glaciated, are today followed by the Nisqually River and Kautz Creek. Thus, the area of a former valley floor is now a ridge. (Fig. 5)] [Illustration: Columns of dark-gray andesite at the east end of an old lava flow from Mount Rainier. This outcrop is near the point at which the highway to Yakima Park crosses Yakima Creek. (Fig. 6)] [Illustration: Layers of pumice on the floor of a cirque near Paradise Park. The yellow bed at the bottom is layer O, which was erupted by Mount Mazama volcano at the site of Crater Lake, Oregon, about 6,600 years ago. The yellowish-brown layer a few inches above layer O is layer D, a pumice that was erupted by Mount Rainier between 5,800 and 6,600 years ago. The light-yellowish-brown pumice bed at the top of the outcrop is layer Y, which originated at Mount St. Helens volcano between 3,250 and 4,000 years ago. Photograph by D. R. Mullineaux, U.S. Geological Survey. (Fig. 7)]