Trip Report: Geology Hike On Cougar Mtn.
Ancient volcanic mud flows, 40 million year old river and swamp deposits that later turned into coal, and a fascinating rockfall that Harvey Manning once called a quarry--all of these and more were covered on a recent IATC hike on Cougar Mtn.
The Red Town trailhead at the edge of the Cougar Mtn. Regional Wildland Park is a place where many of IATC’s interests converge. It’s the jumping off place for miles of trails that weave through the forest and sloping terrain of the park. It sits just across Lakemont Blvd. from the 12-acre parcel of land that the Save Coal Creek conservation effort is working to protect from the bulldozer and shovel. And, as it turns out, it is an excellent place to meet for a hike to explore the geologic past and more recent mining history on Cougar Mtn. For local geologist and teacher Tom Doe, these last two attractions were his “hobby and a diversion during COVID.” Fortunately for the 15 of us who gathered on this cool and overcast Saturday morning, Tom had parlayed his energies into a well-planned and fascinating 3-hour hiking tour.
Pre-Hike Geology and Mining History Presentation
Four days earlier, during an online Zoom meeting Tom gave a presentation on the geologic and mining history of the area, where we were introduced to the fascinating world of Lidar imagery and how it reveals the 12,000+ year-old glacial scrapings of the landscape that are difficult to detect when just walking the land. We also learned about the Tukwila, Renton, and Blakeley formations, the three primary rock units that make up Cougar Mtn., and how more recent folding, tilting, and faulting of the land has brought these three overlapping rock layers to the surface that are there to discover if you know what to look for. [watch the recording]
“Tom was an incredible source of knowledge”, said Justin Roe, one of the hike participants. “I had a great time listening to him speak about something he is obviously so passionate about.”
Hikers out of Red Town have a much easier time self-discovering the mining history of the area due to the many signs and markers placed about. During the presentation we learned that all the coal seams within Cougar Mtn. are found in the Renton formation, the 35-40 million year-old middle rock unit made of sandstone from deposits of ancient rivers, lakes, and swamps. Though mining activity ended decades ago, one of the coal seams makes its presence known because of a decades-old fire that still burns underground today. The last known flareup to reach the surface was about 15 years ago when some smoke and steam rose out of the ground.
The Route
At just under 3 miles long but taking the full 3 hours of allotted time due to the many stops, this wasn’t a physically strenuous hike, but that’s not the primary point of IATC “Hikes with a Purpose” such as this one. Instead, more emphasis is put on the route and the opportunities to learn along the way. On this day we started with a short excursion onto the Coal Creek Trail, then back across Lakemont Blvd. to the Bagley Seam Trail to the Red Town Trail to the Indian and Quarry trails, and back to the trailhead via the Red Town Trail. We finished with a short side trip up a small stream to view a narrow coal seam visible in the streambed.
Harvey’s Quarry
The oldest rock unit, the Tukwila was formed by ancient volcanic mud flow deposits, or lahars. This harder rock is more resistant to erosion than the sedimentary Renton formation that formed atop it before they were both tilted and thrust to the surface by tectonic forces. The main stem of Coal Creek flows south until it encounters this harder rock where it is forced to take a right turn and flow westerly along its edge and then more northerly on its way into Lake Washington. The De Leo Wall, the object of another IATC conservation effort is part of this same volcanic deposit. At the turnaround place for our hike, amid the jumble of large, moss and fern-covered sharp boulders, Tom laid out three possible explanations for this broken landscape that Harvey Manning had once conjectured was a quarry. Are they erratics, or large rocks deposited by retreating glaciers? Not likely. Are they debris leftover from a demolition company that once had practiced their craft against the abandoned concrete structures of former mine openings. Again, not likely. Instead, Tom believes that these massive rocks broke from the larger formation because of seismic activity. His argument was convincing and one that was fun to explore as some of us scrambled through and around and atop what felt like a natural playground.
Natures Way
The final stop of the day offered a glimpse of what early explorers of the area would have seen more of before the mining operations broke ground. A lesser-used side trail just off the parking area led us over a granite glacial erratic, under a downed tree, and finally to a small, shallow stream. At about 2 feet wide and partially submerged, at first the dark material looked like just another decaying tree. But in fact, we were looking at the remains of countless trees and other plant material that once were buried thousands of feet underground where high pressure and millions of years turned it into coal. Too narrow of a seam to be economically viable for mining and because of the visionaries who labored to preserve this land as wilderness, this coal’s fate instead will be determined by the power of a small stream and not the machinery of man.
“Our family thoroughly enjoyed the Geology Hike with Tom Doe. We regularly hike on Cougar Mountain, but we saw things in a new way”, said Roseann Popa, who was accompanied on the hike by her husband and son. “We are looking forward to joining more Issaquah Alps Trails Club outings in the future!"
IATC is grateful to Tom Doe for volunteering his time and sharing his passion for geology and history to make this memorable hike possible. Thank you, Tom!
The professor Tom Doe (left) and his students.
North Fork Falls with glacial till on top and sedimentary rock at its base.
At the entrance to the Ford Slope mine.
Inspecting Harvey’s “quarry”, or Tom Doe’s “seismogenic” rockfall
Exposed coal seam in the bed of a small stream