I've used a number of resources in putting this trip together, particularly the three following publications.

• Roadside geology of Mount St. Helens National Volcanic Monument and Vicinity, by Patrick Pringle (1993), Washington DNR publication.
• Geological Field Trip Guide Mount St. Helens Lava Tubes, Washington, by Jack H. Hyde and Ronald Greeley (1973) (Field Trip number 7 from a publication for which I unfortunately don't have the reference)
• Road Guide to Volcanic Deposits of Mount St. Helens and Vicinity, Washington, by Michael P. Doukas (1988), USGS Bull. 1859.

• 55-43 million years ago - a section of the basaltic ocean floor got wedged against (accreted to) North America and is now known as the Crescent and Siletz terrains.  Afterward, the shoreline was near route I-5 today or slightly to the west.
• 42-17 million years ago - a wide band of volcanoes formed a mountain range referrred to as the Western Cascades (because they were west of the present Cascade Range in Oregon). After millions of years of erosion, only remnants of the Western Cascades exist today.  One of these remnants consists of some basaltic rocks.  These rocks are the remains of some shield volcanoes that formed between about 42-32 million year ago).  These basaltic rocks are called the Goble volcanics.
• 17-12 million years ago - Cascade-style volcanism pretty much shuts down in the area.  The Western Cascades cease to build and begin to erode away.
• 17-5 million years ago - Huge volumes of very fluid basaltic lava erupt from fissures in eastern Oregon/Washington and form the Columbia Plateau.  There are no Columbia basalts in the vicinity of Mt. St. Helens.  This region must have been topographically high at the time the Columbia basalts were erupting.  (Lava is like any fluid and flows in low spots).
• 12-5 million years ago - still no major Cascade-style volcanism, but some small amount of volcanism is occurring as evidenced by intrusions (dikes and sills).
• 5 million years ago to the present - formation of the present-day High Cascades starts about 5 million years ago, but the individual volcano known as Mt. St. Helens really starts growing about 50,000 years ago.  Mt. St. Helens is a geologically young mountain.  The mountain didn't form all at once, but grew as a series of eruptive stages separated by dormant intervals.

Mt. St. Helens is a "composite volcano", which means that it erupts a variety of materials.  Most of that material is "andesitic" or "dacitic" in composition.  Most composite volcanoes produce basalt early in their "lifetimes" and then produce more silica-rich lavas (dacitic and andesitic) as time progresses.  Mt. St. Helens is unusual in that is has produced a fair amount of basalt recently (around 2000 years ago).  During its many eruptions, Mt. St. Helens has produced domes of solid rock (when thick lava oozes slowly to the surface); lava flows that have solidified to form rock; columns of ash erupted into the air; pyroclastic flows (mixtures of hot ash and dense gas that flow down the sides of the mountain); and lahars (debris flows that result from water mixing with ash and rock debris).  Both the columns of ash and the pyroclastic flows deposit loose fragmentary material which is called tephra.

• 50,000-36,000 years ago - Ape Canyon Eruptive Stage: The first eruptive period of Mt. St. Helens.  Deposition of tephra set C.
• 20,000-18,000 years ago - Cougar Eruptive Stage:  Pyroclastic flows and dacitic tephr filled the Lewis River to a depth of approximately 400 feet.  Deposition of tephra sets M and K.
• 13,000-10,000 years ago - Swift Creek Eruptive Stage: Characterized by the euption of large volumes of tephra (tephra sets S and J).
• 9.000 years ago to the present - Spirit Lake Eruptive Stage:  The most recent eruptive stage is divided into seven eruptive periods.  These are 1) Smith Creek (3900-3300 years ago - tephra layer Yn); 2) Pine Creek (2900-2500 years ago - tephra set P); 3) Castle Creek (2200-1600 years ago - tephra set B, eruption of the Cave Basalt, eruption of the andesite flow filling lava canyon); 4) Sugar Bowl (age uncertain, but around 1000 years ago - formation of Sugar Bowl dome and East dome); 5) Kalama (from 1480AD to the late 1700s, so 300-500 years ago - tephra sets W and X, formation of the pre-1980 summit dome); 6) Goat Rocks (1800-1857AD, so around 150-200 years ago - tephra set T, formation of Goat Rocks dome; and 7) Current eruptive period (1980-present).