Earliest eruptive behavior of Ancestral Mount St. Helens dates back to 40,000 years ago but the major growth period of the system is only about 2500 years old (Mullineaux and Crandall, 1981). As early as 1835, Mt. St. Helens was understood to be a volcano and was first viewed by the geologist James Dwight Dana from Yale Univ. (Tilling et al., 1990). The expedition described "cellular basaltic lavas".
The Cave Basalt in which the tubes have formed (Greeley and Hyde, 1972) is a ropy pahoehoe type flow, with high-alumina chemical composition. Its source appears to have been the southwest flank of Mount St. Helens (Hyde and Greeley, 1973). The primary flowpath proceeded approximately 11 km down an existing stream valley that incised into older pyroclastic flows and lahar material of apparent later Quaternary age. The flow ends atop the Lewis River's north bank. River gravels underlay the basalt especially around the river area. Large slabs of basalt at various angles abound, interspersed with flat areas and rolling surfaces. Large cracks occur in many of the pressure ridges and tumuli on the flow.
The dating of charcoal from roots in Lake Cave and Ape Cave areas were dated by radiocarbon technique at around 1900 years old (Hyde and Greeley, 1973). Charcoal samples from two localities under the lava tubes yielded Carbon-14 ages of 1,860+/-250 and 1,925+/-95 years (Doukas, 1990). However, the oldest dated volcanic deposit at Mount St. Helens is a lahar that contains sufficient wood to allow radiocarbon dating (Hyde and Greeley, 1973). The flow is 36,000 years old at the level of the river. An age of 20,000 years was obtained from charcoal in pumice flows overlying the gravels in the Lewis River Valley. Signifcant volcanic ash was produced in 1857 and again in the famous 1980 eruption (Brantley, 1994). For at least the past several hundred years of observation, pumice and pyroclastic eruptions and lava flows have occurred frequently and have often been quite violent.
The Mount St. Helens watershed drains about seventeen square miles and appears to divert all of that water below ground in the Cave Basalt Flow. Sinking streams, or simply precipitation infiltration through fractures makes surface drainages rare. Two main resurgences are large springs at Dry Creek and Cougar Creek.
Dye traces from upper Cougar Creek at Grass Lake three miles underground to Cougar Creek Spring have reached breakthrough in just under three days (Oregon Grotto, 2004).