New petrographic and chemical data indicate that the great Mount Mazama eruption at Crater Lake, Oregon, about 6600 years ago was the source of most ash which has been called "Glacier Peak" and of some ash called "Galata." Glacier Peak volcano in Washington was itself the source of an older ash deposit, perhaps very late glacial or early postglacial in age.

No abstract

Pumice layers of set S from Mount St. Helens can be correlated with certain ash beds associated with young flood deposits of the channeled scabland. The correlation points to an age of about 13,000 14C yr B.P. for the last major flood to have crossed the scabland. Until recently, the last major episode of flooding was thought to be closer to 20,000 yr B.P., an age inferred chiefly from the relation of the flood to glacial events of the northern Rocky Mountains. Several investigations within the last few years have suggested that the last major flood occurred well after 20,000 yr B.P. Tentative correlations of ash beds of the scabland with set S pumice layers, the relations of flood and glacial events along the northwestern margin of the Columbia Plateau, and a radiocarbon date from the Snake River drainage southeast of the plateau all indicate an age much younger than 20,000 yr. The postulated age of about 13,000 yr B.P. is further supported by a radiocarbon date in the Columbia River valley downstream from the scabland tract. Basal peat from a bog on the Portland delta of Bretz, which is a downvalley deposit of the last major scabland flood, has been dated as 13,080 ± 300 yr B.P. (W-3404).

Biotite-bearing chalky-white rhyolitic ash, here called the Bishop ash bed, occurs in-middle Pleistocene alluvial and lacustrine deposits at eight localities scattered from California to Nebraska and is correlated with the basal air-fall lapilli of the Bishop Tuff, an ash flow of eastern California, radiometrically dated about 0.7 million years. Correlation of the Bishop ash bed with the air-fall lapilli is made on the basis of similar petrography and on chemistry as determined by electron microprobe, atomic absorption, and emission spectrographic analyses. At five localities the Bishop ash bed lies stratigraphically below a Pearlette-like ash. As more occurrences of the Bishop ash bed are found, it should become an increasingly important dated stratigraphic marker relating middle Pleistocene deposits and events across several geomorphic provinces.Another biotite-bearing chalky-white ash, here called the ash of Green Mountain Reservoir, occurs at three other localities and is distinguishable from the Bishop ash bed by small differences in chemical composition of the glass. The ash of Green Mountain Reservoir is younger than the Bishop ash bed, as shown by the fact that at one locality it lies stratigraphically above the aforementioned bed of Pearlette-like ash.

Radiant flux. The flow of radiated heat per unit time. Stratigraphic. Pertaining to layered deposits of rock. Tephra. General term for airfall debris produced by a volcanic eruption. Includes but is not limited to ash, pumice, and bombs. Thermal area. Spatial area of thermal ground. Thermal ground. In this report, ground heated by a volcanic source. Water equivalent. The amount of water that would result from the complete melting of a body of deposited snow, firn, or ice, expressed as thickness (depth) or volume.