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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
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