Geology field trip in the Puget Lowland

April 6, 2008

Continental glaciations dramatically sculpted the landscape of the Puget Lowland between about 22,000 and 15,000 years ago. The Puget Lobe of the continental ice sheet reached to south of Olympia. Rivers draining the southern Washington Cascade Range were shunted around the toe of this lobe, merged with meltwater from the glacier, and flowed out via the Black Lake spillway, near South Puget Sound Community College, and out to the Pacific Ocean by way of the Chehalis River valley. Postglacial erosion and deposition has continued to alter the landscape. We will see evidence of glacial and interglacial processes during our trip.

Today we will travel north through the Puget Lowland to see features and deposits that resulted from glaciation-related processes and to look for evidence of the past activity of Mount Rainier. We will also stop at outcrops of bedrock.

Tentative Field stop plan: Leave Centralia College, go downtown and then NORTH on Tower St. and follow State Route 507 (SR 507) north. About 1.5 mi north of Bucoda near the Frost Prairie Store turn right on ?SkookumChuck Road. We will head east several miles to Stop 1. Go back to SR 507 and head north into Tenino. Stop 2 is at Dave’s Market to look at the Tenino Sandstone (stone grocery bag sculpture) and boulders on the southeast side of SR 507. Leave market and go north toward Olympia on old Highway 99. After about 3 miles turn right on Offut Lake Road. After a mile and ¼ or so turn left at the Offutt Lake Sign and park near the Offutt Lake Market Stop 3. Go back to HWY 99 and turn right. Go north ~1.5 mi past Mima Mounds to Waldrick Road and turn right. Keep going 6 – 7 mi and turn right on Military Road. Go back across Deschutes River past McIntosh Lake. Carefully turn left onto SR 597 and head to Yelm. At Yelm, turn right onto SR 510. Keep going straight through at Five Corners (Chevron) and cross the Nisqually River. At the Y turn right onto SR 702. Go 10 mi east to SR 7. Turn right. Head south. When we descend into the Ohop Valley past buff to gray sediments on the right pull off to the right at the very bottom of the road just before it curves to the east. Carefully turn around here in the large turnout area, get back onto the highway headed back up the hill, and turn into the pulloff at the top just before the road makes a sharp left turn: Stop 4. Carefully get back on the highway and go north less than 2 miles to Stringtown Road—turn right and go several mi turning right on Eatonville Cutoff Road. After a short distance carefully turn left and go north on SR 161. After about 6 mi, turn right on 304 ST. E and head toward Kapowsin. At the junction (Orting Kapowsin HWY) go straight past Lake Kapowsin (right) and pull off to the right on the gravel road past the creek at Electron: Stop 5. Leave stop 5 and continue north. Carefully turn left at SR 162 and head into Orting. Stay on the main road as it winds through town and turn left into the Safeway complex for a break. Stop 6 is Ptarmigan Middle School, ~1/2 mi north of town on the left. Leave school and carefully turn left. Go north ~ 7 mi nearly to Sumner and turn right on SR 410. Follow SR 410 all the through Buckley and Enumclaw. Go east 4 to 5 mi and turn right at Mud Mountain Dam. Stop 7: Restrooms may be open at the viewpoint (~2 mi in) however note the big parking area on the left ~ ¾ mi in from SR 410. This is where we will park for our hike into the canyon after visiting the viewpoint. Return to Olympia via SR410, 512, and I-5 going through Puyallup.

While driving on Pacific Highway SW we will pass through Violet Prairie, which in many areas is covered by Mima Mounds. The Mima Mounds are one of geology’s mysteries that have never been convincingly explained. What could have caused these mounds?

1. At the conclusion of this trip, search in the library and on the internet and find at least three sources of information on the Mima Mounds. An important library resource is http://www.dnr.wa.gov/geology/washbib.htm What did you find out? Remember to list the citations you found. Ask you instructor for hints on where and how to search for this information. In your field notebook draw a sketch of the mounds and make a few observations about their dimensions, spacing, etc. Speculate on how you think they might have been formed.

If we stop at the Mima Mounds note the size range, shape, and sorting of sediments in the mounds. Also note their overall size, shape, and distribution. We will get another look at the Mima Mounds if we visit the Mima Prairie Natural Resource Conservation Area managed by the WADNR.

At the junction of State Route (SR) 507, continue north through Tenino making sure you are going the speed limit! At Pacific Highway urn left into the parking lot at Dave’s Grocery store.

You have probably noticed some of the sandstone buildings as you passed through Tenino. The Tenino quarry is near here, and was a source of sandstone for the old State Capitol building in Olympia, the Thurston County Courthouse, and other structures. In the parking area is the sculpture of a grocery bag in Tenino Sandstone. We will look at the grocery bag to describe the texture of the sandstone and any other features.

2. Describe the sandstone here according to the size and mineralogy of the grains and sorting.

Notice the boulders across SR 507 from the grocery. Tenino is littered with boulders like this. Many are composed of andesite rock, the kind that originated at Mount Rainier. Are the boulders rounded or angular? What kind of processes could have carried these boulders here?

Depart the grocery and head toward Olympia on Old 99, Pacific HWY SE. Turn on Offutt Lake Road and follow the signs to Offutt Lake. We will park at the Offutt Lake store and walk a short distance to the lake.

3. At Offutt Lake boat launch area notice the many boulders? Closely examine the boulders in the fireplace area or nearby. What do you notice about their texture? How would you describe this? Are the boulders diverse in rock type or do they look like they are of a similar rock type?

After returning to Offutt Lake Road, turn right go back to OldHWY 99 and go north to Waldrick Road. Turn right and follow the Deschutes River upstream to Military Road SE and turn right, crossing the valley of the Deschutes River.

4. Notice the large rounded gravel bar on the “left side” of the river valley. (“river left means on the left side as you look downstream). Can you see any evidence of the size of the particles making up this bar? What about the overall size of the bar? Does that provide a clue to the hydraulics and discharge of the flow that deposited it?

After this trip, consult the recently completed geologic map of SE Olympia (GM-56) by Robert Logan and Timothy Walsh to learn more about the geologic history of this area. What do you notice about the many terraces they mapped? This map will be in the library and is also available online at http://www.dnr.wa.gov/geology/pubs/pubs_ol.htm

Continue in a SE direction on Military Road to SR 507. At the highway, carefully turn left and head toward Yelm via Rainier, making sure to go the speed limit. Note any boulder fields or other features you see along the way.

5. Between Rainier and Yelm, note the undulating terrain? Note the sediment make up. Could these features be large bars? What is the approximate spacing of the features?

At Yelm, turn right on 507/510 and continue over the Nisqually River, which heads on Mount Rainier. Turn right on SR 702 and head east to SR 7.

Note the flat terrace at the intersection of 507 and 702. Terraces can record a river’s history. This terrace is veneered by a sand derived from a lahar from Mount Rainier. This “lahar runout” is the diluted flood from a lahar, a far-traveling volcanic debris flow. Sandy lahar-derived flows such as this one are commonly triggered by eruptions that melt snow and ice. The age of this deposit is estimated to be about 2,450 yr. B.P., and its sediments have been found in the Nisqually River delta. Thus, the volcanic flood flowed all the way from Mount Rainier to Puget Sound.

Continuing east, the road goes up and down over N-S-oriented elongate ridges the are interspaced with parallel valleys or “flutes”. This is classic glacially sculpted terrain. The hills are drumlins, or streamlined molded ridges formed by the movement of glacial ice over sediments. The orientation of the axes of the ridges shows the direction of movement of glacial ice.

At SR 7, turn right and go south on SR 7. The road descends into the Tanwax Creek valley, which was cut into glacial deposits by great floods of water. .

Note the boulders in this area. These large boulders are mostly andesites, and the Tanwax valley connects with the network of valleys downstream that contain andesite boulders. This is evidently the pathway of a large flood at the end of the Ice Age. The flood originated with lakes that were dammed up by the ice along its margin.

Continue on and after a sharp right turn descend into the large Ohop Valley past buff to gray sediments on the right pull off to the right at the very bottom of the road just before it curves to the east. Carefully turn around here in the large turnout area, get back onto the highway headed back up the hill, and turn into the pulloff at the top just before the road makes a sharp left turn: Stop 4.

We will very carefully walk along the edge of the highway a short distance to see the deposits on the west side. DO NOT CROSS THE HIGHWAY. KEEP A SAFE DISTANCE OFF THE ROAD AND WE WARY OF TRAFFIC. Sketch the outcrop here and describe the deposits. Note clast size, shape, and sorting. Speculate on what type of natural process might leave these deposits. The uppermost two layers, here and several others below are Pleistocene in age. Below these deposits much of the exposed material in the gray outcrops along the road is the Miocene age Mashel Formation, which mostly consist of material deposited in river valleys and lakes and which are full of pumice and dacite fragments similar to the type of lava being erupted by Mount St. Helens today.

Carefully get back on the highway and go north less than 2 miles to Stringtown Road—turn right and go several mi turning right on Eatonville Cutoff Road. After a short distance carefully turn left and go north on SR 161. After about 6 mi, turn right on 304 ST. E and head toward Kapowsin. At the junction (Orting Kapowsin HWY) go straight past Lake Kapowsin (right) and pull off to the right on the gravel road past the creek at Electron: Stop 5.

6. Lake Kapowsin contains a submerged forest. The lake was formed about 500 yrs ago when it was dammed by the Electron Mudflow from Mount Rainier. The Electron Mudflow was named for the town of Electron, where US Geological Survey geologist Dwight “Rocky” Crandell found a log in it that he dated using radiocarbon. We might stop quickly here to examine the mudflow deposit. Describe the sorting of the sediment. Can you tell how much clay it contains? Explain.

Proceeding north on Orville Road, we will pass an outcrop of the Orting Drift, the oldest known glacial deposit in the Puget Lowland. We continue north across the Puyallup River bridge and the valley widens as we approach Orting.

7. Notice the overall convex shape of the valley bottom here. The entire valley is covered by the Electron Mudflow, which has a depth of more than 20 ft. here. Crandell measured one large boulder the mudflow had carried down at 32 ft in length! What does clast size imply?

Turn left on SR 162 and continue to Orting –go the speed limit. We can stop in town for coffee or restrooms. Pull into the north end of the Safeway Parking lot on the left where we will have a short break.. After the break, we will go a short distance north and turn left into the parking lot at Ptarmigan Middle School.

Here we will see some large remnants of the subfossil trees that were found beneath the ground. The trees, which were buried by the Electron Mudflow, were rooted about 16 ft below the surface. Geologists estimated that the lahar reached here from Mount Rainier, about 30 miles flow distance, in about one hour or less.

8. What are the implications of future similar events? Sketch a cross section that shows rooted trees 16 ft below the surface. Why are none of the trees protruding above the surface today?

Option: We may continue to Mud Mountain Dam. If not continue north from the school on SR 162. We will take a left on Military Road and go up the hill continuing over to SR 161 (Meridian) Turn right and go north to SR 512. Take SR 512 to the east to I-5. Take I-5 south and get off at the exit 105-B. Continue north; we well follow East Bay Drive nearly all the way to Burfoot Park.

OPTIONAL TRIP TO BURFOOT PARK

9. Burfoot Park (sketch and interpret) At the beach we will see several fascinating exposures of glacial and interglacial deposits. Draw a cross section that depicts and describes the layers—provide scale. What is the nature of the lavender colored sand? Sketch and describe the layers and types of rocks you find on the beach (at least 10). END!

2^ND Optional stop.. I-5 south to the Littlerock exit. West to the T-intersection ~3.7 mi, then right a short distance to Mima Mounds State Preserve. (DNR quarry is accessed by going left at this intersection and heading south about 1.2 mi and then heading ¼ mi west on Bourdeaux Rd. There mounds can be seen in cross section. Sketch and interpret if we go there.