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)
D. R. Mullineaux of the U.S. Geological Survey has studied in detail the pumice deposits of Mount Rainier National Park. One of his first and most important discoveries was that even though some pumice layers are spread widely over the park, they were erupted from other volcanoes. Strangely enough, one layer is thicker and more widespread than any recent pumice erupted by Mount Rainier. We can clearly see that these foreign pumice layers did not come from Mount Rainier, for they thicken and coarsen southward, away from the park. The oldest was erupted by Mount Mazama volcano at the site of Crater Lake, Oregon, about 6,600 years ago; this pumice forms a yellowish-orange layer about 2 inches thick nearly everywhere in the park. The pumice has a texture like that of sandy flour, and it feels grainy when rubbed between the fingers. It is so fine grained because of the great distance to its source, 250 miles due south of Mount Rainier. Near Crater Lake this same pumice consists of large chunks and is many feet thick.
Two other foreign pumice deposits in the park were erupted by Mount St. Helens, a symmetrical young volcanic cone about 50 miles southwest of Mount Rainier. The older of the two is between 3,250 and 4,000 years old; it forms a blanket of yellow sand-sized pumice that is as much as 20 inches thick in the western part of the park. The younger pumice layer is most conspicuous at the ground surface in the eastern part of the park, where it is as much as 4 inches thick and resembles a fine white sand. It is about 450 years old.
Mount St. Helens as it appears from Mount Rainier.
An inconspicuous bed of pumice records the first eruption of Mount Rainier that occurred after Ice Age glaciers melted back to the slopes of the volcano. It can be found on the east side of the mountain from Grand Park south to Ohanapecosh campground ([fig. 8]). In roadcuts near the east end of Yakima Park (Sunrise) the pumice forms a rusty-brown bed about 4 inches thick which contains fragments as much as 2 inches across. Wood from a thin layer of peat just above the pumice was dated by its content of radioactive carbon as about 8,750 years old; thus, the pumice is even older. We call this pumice layer R for convenience; other letter symbols have been assigned to the younger layers ([table 1]).
Generalized distribution of some pumice layers within Mount Rainier National Park. The pumice of layers W and Y was erupted by Mount St. Helens; all the other pumice originated at Mount Rainier. Letters represent the following localities: C, Cougar Rock campground; I, Ipsut Creek campground; L, Longmire; M, Mowich Lake; O, Ohanapecosh campground; P, Paradise Park; S, summit crater; T, Tipsoo Lake; W, White River campground; and Y, Yakima Park. Based on studies by D. R. Mullineaux. (Fig. 8)
Layer X (Between 110 and 150 years old)
