Supplemental field trip information & maps – 15 Mile Creek
Geology & Lidar Map of 15 Mile Creek
Cenozoic Stratigraphic Columns, Armentrout et al. (1983)
Geologic Map by Vine (1969)
Puget Group Slides for Discussion

Supplemental field trip information & maps – Talus Parcel 9 Landslide
Geology & Lidar Map of Talus Parcel 9 Vicinity
Lidar Hillshade Maps of Talus Parcel 9
Orthophotographs of Talus Parcel 9
Geologic Map by Booth et al. (2012)
Rosengreen (1965) Thesis Excerpt

What to bring
Hiking boots and rain gear recommended. Bring your own water, sack lunch, snacks and a rock hammer. Leashed, well-behaved dogs are welcome.

Initial Meetup Location
~9:45 am – Park at the Rainier Trail entrance, next to the Issaquah Dog park.
Walk ~1000 feet south along the Rainier Trail to the field trip gathering spot at the Jakob Two Trees sculpture, where we’ll kick off the field trip at 10:00 am with an overview geology discussion.
Don’t be late, as our next stop (Stop 1) will take us through a locked forest gate, and you’ll be locked out.

Stop 1: Fifteen Mile Creek
We will explore the Tiger Mountain amber collection site along the east slope of Fifteenmile Creek. The site consists of gray shale, siltstone, and sandstone in a south-striking dip slope (Ellis, 1961). Vine (1969) maps the outcrop as Tukwila Formation, a volcaniclastic-rich unit of the Puget Group with andesitic lava flows and tuffs dated between 39 to 40.5 Ma (Nesbitt, 1998). The Tukwila Formation is intercalated between the marginal marine / fluvio-deltaic underlying Tiger Mountain Formation and overlying Renton Formation (Nesbitt, 1998). Mustoe (1985) re-designates the outcrop as Tiger Mountain Formation and notes that the surrounding sub-bituminous coal indicates low thermal alteration. We will see at least one coal mine portal on this hike. Amber is fossilized tree resin. The amber at this site is assumed to be coniferous based on associated plant macrofossils (Waggoner, 1993). Collection in 1961 yielded a pint of amber in 1-2 hours (Ellis, 1961). However, a WGS field trip in 2019 yielded only one ~1 cm piece from 20 people searching for half an hour. We’re crossing our fingers that the recent winter weather has unearthed some additional amber for us! In addition to searching for amber, we will discuss the broader depositional environment, tectonic deformation, and historical coal economy of the area.
References:
Ellis, Ross C., 1961, Early Tertiary amber near Seattle, Washington: [Privately published by the author], 3 p.
Nesbitt, Elizabeth A., 1998, Marine fauna of the middle Eocene Tukwila Formation, King County: Washington Geology, v. 26, no. 1, p. 13-19.
Vine, J. D., 1969, Geology and coal resources of the Cumberland, Hobart, and Maple Valley quadrangles, King County, Washington: U.S. Geological Survey Professional Paper 624, 67 p., 4 plates.
Waggoner, Benjamin, 1993, Fossil actinomycetes and other bacteria in Eocene amber from Washington State, USA: Tertiary Research, v. 14, no. 4, p. 155-160.

Stop 2: Talus 9 Landslide
The Talus 9 landslide in Issaquah, Washington occurred in November 2015 following wet-season construction of an approximately 15-foot-thick fill prism on the head of an ancient landslide. The construction was related to residential development of Parcel 9 of the Talus subdivision in Issaquah, WA, located on the east side of Cougar Mountain. Reactivation of the slide coincided with a period of prolonged heavy rainfall and the rupturing of a city water line. The movement caused significant ground deformation, including buckling and cracking of roads near Talus Drive and Shangri-La Way NW, ultimately halting construction in the area. Initial geotechnical investigations attributed the failure to a combination of excessive groundwater saturation, preexisting weak soil conditions, and construction-related modifications to the slope. Emergency stabilization efforts included expedited removal of the fill, dewatering wells, erosion controls, and temporary retaining structures.
Subsequent geotechnical studies further analyzed the mechanisms of failure, recommending long-term stabilization strategies to mitigate future movement; these included grade beams and long tiebacks, which appear to be working as intended. A legal battle between the city of Issaquah and the developer was ultimately settled out of court. Since then, ongoing monitoring and mitigation measures have aimed to permanently stabilize the slope and assess the risks for future development at the site. The Talus 9 landslide provides a case study in the challenges of hillside construction and the importance of robust geotechnical design in landslide-prone environments.

Post-trip meetup at Formula Brewing (optional, unofficial stop)
Join us after the field trip for an unofficial gathering at the family-friendly, dog-friendly Formula Brewing in Issaquah.