Western Snake River Plain: Field Trip at Idaho Museum of Mining and Geology – Boise, ID

We will meet in the Old Pen gravel lot, 2445 Old Penitentiary Rd. Boise, near the trailhead sign at 7:45 am for an 8 am departure. This site will also serve as the first stop. We will not be hiking up the trail, just meeting there for the first stage.

Throughout the morning there will be minimal walking—no more than a few hundred yards. (Drive, talk, walk…repeat)

Steve Schaps and Tom Frey will facilitate registration and carpools.

We will visit 4 field sites in the Boise foothills near Table Rock to study the rocks, geologic features and their relationships. The geology we see at these sites is representative of the geology throughout the western Snake River Plain:

Site 1 at the Old Pen. From this vantage point, we can see Table Rock and the upper ridge consisting of the Terteling Springs formation (Lake Idaho sandstone). The 9-million-year-old Boise Basalt Assemblage underlies the Terteling Springs formation and the 11.8-million-year-old Quarry View Park rhyolite underlies the basalt on the ridge. A geothermal aquifer is located at this site. One of the Boise Front faults has fractured the rhyolite and the aquifer is confined by basaltic tuffs.

Site 2 at North Alto Via and Table Rock Road. This site offers an overview of the Western Snake River Plain, the Owyhee front, the Boise Valley, the Pleistocene terraces, and the Landslides of Table Rock. The western plain is a down-faulted basin or graben filled with 2 miles of lakebed sediments and basalt; the Owyhee Mountains are a succession of linear time-progressive caldera eruptions; and the Boise Valley began to form when Lake Idaho terminated 2 million years ago. This is also the site of the 2016 North Alto Via slump that destroyed 6 homes.

Site 3 at Eastdale Drive. This site has an excellent example of two water-affected basalt flows erupted along the bottom of Lake Idaho. These two flows are distinctly different and are separated by a red baked zone. A nearby house is built on a basaltic tuff that overlies the two basalt flows of the 9-million-year-old Boise Basalt Assemblage. The basalt is capped by a remnant of Terteling Springs sandstone

Site 4 on Shaw Mountain Road/Cottonwood Creek. Site 4 offers the same stratigraphic relationships as the other sites but also has the 70- to 80-million-year-old Idaho batholith in contact with the 11-million-year-old Cottonwood Creek rhyolite. They were brought together by large movement along one of the Boise Front faults. The Cottonwood Creek rhyolite is thought to be a lava flow rather than an ash flow tuff.

Each participant will receive a detailed field guide of the field trip, emailed a couple days before the trip.

Members of Idaho Museum of Mining and Geology- Free, Nonmembers of the Museum-$20. Everyone must pre-register. https://www.idahomuseum.org/events/category/field-trips/

Dress for the weather and bring water and snacks. Be sure your email address is correct on your registration. Check your email the day before in case the trip is canceled.

No dogs, please.

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About Dr. Terry S. Maley:

Dr. Maley is an esteemed and sought-after lecturer. He has been an avid field observer, photographer, and illustrator of geological features during more than 50 years of fieldwork. Now retired, he worked many years for the Department of the Interior, conducting geologic investigations throughout the western United States. Before joining the Interior Department, Terry was a marine geologist of the U.S. Naval Oceanographic Office for eight years, participating in 15 worldwide expeditions. He also served as Administrator for the Idaho Division of Earth Resources for five years with responsibility over all mineral resources, water resources, and geological research in Idaho. He has authored more than 50 technical papers on geological features and has written ten books on regional geology, field geology and mineral law. Dr. Maley’s book, Exploring Idaho Geology, is sold in the museum Gift Shop.

Please contact operations@idahomuseum.org if you have questions.