Purpose of This Sheet: To locate the attractions and explain the geology encountered along SR 504, the showcase highway to MSH named Spirit Lake Memorial Highway but sometimes called the West Side highway. Details of most attractions are found on other sheets, particularly 3a and 3c. Geological features are identified “Geo-”. This information comes from Roadside Geology of Mount St. Helens National Volcanic Monument and Vicinity, Patrick T. Pringle; WA DNR, 1993, 120pp.
MP=Mile Posts: White metal plates with green numbers mounted on short metal posts placed along south side of highway at one mile intervals measuring from middle of I-5. Watch for them when you turn off I-5. At times we add tenths, i.e., MP 33.5, to further help in locating an attraction but the markers are only found each mile.
MP 0.0 The center of Interstate 5 at Exit 49 (State Route 504 also called Spirit Lake Memorial Highway). Services are found on east side of interchange. Cinedome theater in blue high on the north side of SR 504 shows a 25 minute film, The Eruption of MSH, on a giant screen every 45 minutes beginning at 9 AM, May-Oct. The theater has “eruption” seats to let you feel the power and experience the eruption. Adults-$5; children/Seniors-$4. Geo-you are on an ancient lahar (volcano caused mudflow) overlain by deposits from the Lake Missoula flood. The road climbs the “wall of the Cowlitz River valley” and takes you over deposits thought to be from large valley glaciers extending from Mount Rainier. A quarry at MP 2.5 exposes a lava flow.
MP 5.3 Elevation 500’. MSH National Volcanic Monument Visitor Center (also called Silver Lake Visitor Center; see article in 3c-West Side Volcano Attractions.) and Sequest State Park (MP 5.6) with picnic tables and 76 campsites. Geo-Silver Lake is on your right for the next four miles. It was formed by a lahar many years ago.
MP 9.5 MSH Creation Information Center. (See brochure and “Memorial Stones” story.)
MP 10.2 Toutle, unincorporated. Only town east of I-5 on SR504. Silverlake is not a town. Toutle Lake School (750 students, K-12), post office, Drew’s Grocery and gas, Toutle Diner, Toutle Lake Church.
MP 11 Toutle River bridge (Coal Banks bridge). Geo-the 1980 eruption produced a lahar that jammed logs beneath an earlier bridge and lifted it off its piers as officers blocked traffic and a camera man took pictures. The piers were raised and support the present bridge. The two branches of the river merge a short distance up stream. The South Fork Toutle River drains the west side of MSH, while the North Fork Toutle River drains the north and northwest sides. To simplify, I refer to the North Fork Toutle Valley/River or North Fork Upper Toutle Valley throughout these papers as the “Toutle Valley/River” or “Upper Toutle Valley”. The bluff beside the bridge upstream is part of a lahar that formed Silver Lake by blocking a stream. The flat plateau left by the lahar is 60’ above the river. Experts calculate the lahar discharge rate as similar to that of the Amazon River at flood stage, nearly nine million cubic feet per second of rocks, sand, mud, and water.
MP 14.5 Junction with State Route 505 that reaches Interstate 5 at Toledo. It cuts nearly 10 miles off your trip if you are going north on I-5 when you return from your visit to MSH.
MP 19 Nineteen Mile House restaurant, campground, trailer park, etc. A three-generation family business. Excellent food. Last gas. Area called Kid Valley. Mudflow buried 19-Mile Logging Camp across river.
MP 19.6 Maple Flats where the mudflow claimed many homes. North Fork Survivors gift shop. Home of the buried A-Frame. Photo exhibit/free movie. Pacific Air Tours jet helicopter pad.
MP 21 Elevation 900’. Junction with one-mile-long Sediment Dam Road to Sediment Retention Structure (SRS) (see 3c) trail and free private visitor center and gift shop. Next section of the highway opened in 1992. Geo next few miles-geologic outcroppings expose basaltic andesite lava flows and flow breccia.
MP 25 Geo-in a cliff on the north side of the road a series of lahar deposits and ash beds containing fossil wood are exposed as well as andesite and basalt volcanic rock. Note that the rocks are more brightly colored and increasingly cut by dikes and sills as you drive east.
MP 27 Elevation 1400’. Hoffstadt Bluffs Visitor Center (restaurant, gifts & viewpoint). (See 3c.) You are 15 miles NW of MSH. Looking up the valley toward MSH, see the rubble of the first sediment retention dam. It was breached by the lahar of March 19, 1980. Its failure indicated that a much larger structure would be needed to contain future lahars and reduce the extreme sediment loads in the river. This led to the construction of the SRS (see MP 21). Geo-beyond it are the first of numerous hummocks and the lower end of the debris avalanche (See 2b.)
MP 27.8 Geo-columnar jointing in basalt is visible on the left at the east end of the second small bridge after Hoffstadt. Columns form as a result of contraction of the lava during cooling.
MP 29.7 Elevation 2340’. Hoffstadt Creek Bridge & west end viewpoint. Most spectacular of the highway’s 11 new bridges . It is 370 feet above the creek, 2,340 feet long and cost $13 million. Be sure to read the three informational plaques at the viewpoint. This bridge marks the beginning of the blast zone which had an outer ring of trees killed by the searing heat and the inner ring of trees knocked down. However this $165 million highway has one defect. It goes through private timber lands that were harvested before or salvaged after the eruption, so one passes miles and miles of neatly sawed off tree stumps instead of blown down trees.
MP 32.3 & following Geo-notice light-green dikes and multicolored breccia and lava flows in the rock...partially altered to clay minerals such as greenish chlorite.
MP 33.3 Elevation 2670’. Weyerhaeuser Forest Learning Center and Viewpoint. (See 3c.) Geo-Toutle valley is wide and littered with hummocks. The river reestablished itself on the south side of the valley in 1984, deepening and widening its channel, removing much debris-avalanche material.
MP 37 Elk Rock viewpoint. Ten miles from MSH. Elevation 3750’ here and 3800’ at the pass half a mile ahead. Geo-another spectacular view, the best before Johnston Ridge. Elk Rock is really a small mountain which the highway is traversing for about five miles. The visible effects of the eruption become more obvious from this point on, both in the valley and on the slopes. Spud Mountain and Elk Rock frame the mouth to the Upper Toutle valley that slowed the debris avalanche. The decreasing velocity caused a ponding effect at the mouth. Notice from west to east: ponds on debris-avalanche deposit, Spud Mountain, Jackson Creek and empty lake bed, braided channel of the Toutle, hummocks, Pumice Plain, Windy Ridge, Johnston Ridge, Mount Adams, downed trees, Coldwater Peak. MSH National Volcanic Monument is on the right and private timberland is on the left for the next five miles.
MP 38.8 Geo-roadcut exposes rocks-bright greens and pinks cut by numerous dikes. The greenish rocks are an altered pumiceous tuff.
MP 40.5 Castle Lake Viewpoint. Eight miles from MSH. Geo-excellent views of MSH, avalanche deposit and structure of neighboring bedrock valleys. Castle Lake was created by the avalanche which dammed the mouth of Castle Creek. Engineers cut an outlet to keep the lake at a safe level. The Toutle shifts back and forth across the valley floor. Trace its different paths. Find (west to east): Spud Mountain, Castle Lake and Castle Creek, ponds among hummocks, Toutle River, MSH, Coldwater Creek, Pumice Plain, Johnston Ridge, Coldwater Ridge VC, Coldwater Peak. Continue steep descent to Coldwater Lake.
MP 43 Spur up to Coldwater Ridge Visitor Center. Elevation 3200’. (See 3c.)
MP 45 Elevation 2500’. Coldwater Lake Recreation Area with picnic tables, restrooms, boat launch. (See 3c.)
MP 45 South side of highway: 2.3 mile Hummock Trail parking lot. (See 3e.) Geo-the valley ahead is called South Coldwater valley and the stream in it is South Coldwater Creek. The road is built on the lower slopes of the south side of a ridge, makes a sharp turn at the head of the valley and climbs sharply on the north side of Johnston Ridge. Some of the avalanche material on May 18 “topped the saddle between Johnston Ridge and Harrys Ridge, spilled over into the South Coldwater valley near its east end and flowed west down the valley, creating the trimline now faintly visible on the north valley wall. Vegetation and most of the soil below the trimline were scraped off by this first part of the debris avalanche.” -WM (See 3g-Sources). The valley floor is now more than 240’ above the old surface at the west end of the valley and 80’ above it at the east end.
MP 49 Drainage tunnel outlet. Just below the bridge at the head of the valley is the lower end of a 1.6 mile, 11 foot diameter tunnel which maintains the elevation of Spirit Lake at a safe level (3445'). Most of the water in the creek comes from this $13.5 million tunnel.
MP 52 Loowit Viewpoint. Spectacular viewpoint, but the one just ahead is the best of all.
MP 52.5 Johnston Ridge Observatory. (See 3a.) The best trail for seeing the seven wonders (see 3b) begins at the southeast corner of the parking lot which accommodates 350 cars, 50 RVs and 20 buses.
Lloyd Anderson 2/99