As a whole, the set typically is recognizable as a sequence of thin lapilli layers interbedded with the purplish-hued ash. No oxidation profiles were identified within set M. Figure 12. Typical colors of lapilli layers and interbedded ash in tephra set M 9 km east of Mount St. Helens. Mm and Mp in the upper part of the set are separated by brown ash.
Ice axe is shown for scale.
Photograph taken in 1971. Except near its base, set M is not commonly interbedded with ash deposits of flowage origin. Composite columnar section of tephra set M, Mount St. Helens. All named tephra layers in set M contain abundant lithic and pumice fragments. Upward within the set, however, hypersthene increasingly replaces cummingtonite, and ferromagnesian mineral suites of upper layers consist almost entirely of hypersthene and hornblende.