Layer Mm was identified from east to southeast of Mount St. Helens. It is somewhat thicker toward the southeast, and its maximum known thickness is about 15 cm at 17 km in that direction. The layer should be recognizable much farther southeast but was not traced more than about 20 km from the volcano. Pumice in each bed is characterized by ferromagnesian mineral suites of hypersthene and hornblende in which cummingtonite is sparse and in some samples absent. Upper part of tephra set M showing layer Mt, Mount St. Helens.
Pocketknife is at the top of the upper lapilli-bearing bed of the layer.
Photograph taken in 1979. The layer is recognized as thin, lapilli- bearing beds above layer Mm, and by its ferromagnesian mineral suite. The beds are the only known tephras older than set J that contain little or no cummingtonite. The lowest lapilli bed in the layer is underlain by brown ash that separates it from the coarser and thicker layer Mm. The upper bed is overlain by the oxidized, ash-rich deposit that separates set M from set K. Layer Mt was identified at only a few outcrops east of the volcano.