"Tumtum Mountain [is] a small, symmetrical, conical mountain rising 427 m (1400 ft) above the east end of Chelatchie Prairie some 32 km (20 miles) southwest of Mount St. Helens ...
Recent studies have demonstrated that the mountain is a dome-like dacite extrusive body, and dacite petrographically indistinguishable from the dacite forming Tumtum Mountain is also exposed in a gently slopeing area at the north base of the mountain where it overlies glacial drift. The dacite exposed in this gently sloping area is a lava flow and not a debris flow or landslide material as shown by the accordant normal remnant magnetism of samples collected at nine places from the unit. ...
Tumtum Mountain is the youngest, western-most Quaternary volcano in the Cascades of Washington. Because lavas from Tumtum Mountain were erupted during the pre-Vashon interglacial interval, radiometric dating of Tumtum lavas might provide an absolute age for this interglacial interval. ...
In the late Pleistocene, probably in early Wisconsin time, a large tongue of ice came down the Lewis River valley and spread widely across the lowlands and adjacent low hills and benches (Mundorff 1984). Amboy glacial drift deposited by this glacier forms a nearly continuous sheet over all rocks that were overridden by the ice. The ice reached an elevation of about 760 m (2500 feet) 13 km northeast of Tumtum Mountain and about 610 m (2000 ft) 5 km south of Tumtum. Projection of these and other points indicate that the top of the ice sheet probably was nearly 50 m higher thant the present 611 m (2004 ft) elevation of the summit of Tumtum Mountain. If the mountain existed during Amboy time, the ice sheet would have eroded it, leaving glacial drift on its surface. Glacial drift deposited down slope would contain material plucked from Tumtum Mountain by Amboy ice. No glacial drift exists on Tumtum Mountain, and the symmetrical shape of Tumtum indicates it was not eroded by glacial ice. No clasts of Tumtum origin have been identified in glacial drift down gradient from Tumtum Mountain. ...
[Tumtum's] lava flow overlies Amboy Drift along the south rim of Canyon Creek valley. At Location 5 [road cut along south rim of Canyon Creek valley] massive Tumtum dacite overlies a 40-50 cm layer consisting of fragments ot Tumtum dacite in a matrix of fine sand and silt, which covers 9 to 10 m of boulder till (Amboy Drift) which, in turn, overlies Tertiary volcanic rock. ...
The normal magnetic polarity of Tumtum dacite indicates a possible age of less that 700,000 years. Carbon 14 dating of wood fragments from the Amboy Drift indicates an age of more than 60,000 years (Mundorff, 1984). In the Canyon Creek area Amboy Drift is covered by the 35,000 to 50,000 years old (Mullineaux and Crandell 1981) Mount St. Helens ash layer C (Mundorff 1984).
The time interval between Amboy glaciation and the eruption of Tumtum is indicated by weathering rind thickness measured on pebbles from Amboy Drift. At Location 5 pebbles from the upper 0.5 m of Amboy Drift, immediately below the 40-50 cm layer of fragmental Tumtum dacite, were collected for measurement of rind thickness. ... If it is assumed that all rind development on the stones found beneeath Tumtum dacite occurred in the interval between deposition of the drift and extrusion of the dacite, then that interval could have been less than 12,000 years. ...
Evidence that Tumtum Mountain is an extrusive volcanic feature of post-glacial age are summarized as follows:
- The form of Tumtum Mountain, a symmetrical conical peak, indicates it was not eroded by glacial ice even though the loose talus at the surface of Tumtum would have been very susceptible to erosion.
- No glacial deposits are found on the mountain although the pre-Fraser Amboy glacier would have overtopped the mountain had it existed during glaciation, and there are no Tumtum stones found in Amboy Drift down gradient.
- The nature of the rock of Tumtum Mountain, a very fine grained, light buff to gray, platy dacite, is that of a volcanic rock.
- Tumtum dacite overlies Amboy Drift in a gently sloping area of about 0.12 km2 at the north base of the mountain. That this is a volcanic flow unit and not landslide debris is indicated by uniformity of orientation of remnant magnetism throughout the unit.
Tumtum Mountain sits astride, and was probably erupted from, the northeasterly striking Chelatchie fault zone. The fault zone is indicated by prominent scarps, both in surface expression and in the subsurface as shown by well logs, and by wide shear zones which align with the scarps."
[Web author's note: the report doesn't actually list a definitive age. Bolded "headlines" Amboy Drift and Age were added by web author and were not from the original report.]
Maurice J. Mundorff and Albert A. Eggers, 1988, "Tumtum Mountain, a late Pleistocene Volcanic Dome in Southwestern Washington": IN: Northwest Science, Vol.52, No.1.