Lake Walcott Tuff at American Falls Dam
September 5, 2013 Bill Phillips Idaho Geological Survey
Basalt lava flow
red platy welded vitric
ash. Sample 130905-2
Blue-gray spherulitic zone
grading upward into
lithophysal devitrified rhyolite;
v. sparse plagioclase crystals.
Sample 130905-3
Vitrophyre with artifact
quality black obsidian.
Sample 130905-1
Unwelded tuff, white,
planar bedded grading
upward to slightly welded
tuff, greenish gray; overlies
tuffaceous sediments
Location: 42° 46.596’ -112° 52.588’, angler access to Snake River below American Falls Dam.
Geologic map: USGS Bulletin 1121-G, Geologic map of the American Falls quadrangle, Trimble and Carr,
1963. Description Walcott Tuff:
Access: public access by paved road with abundant parking for angler access. Outcrop heavily overgrown
with brush. Contact Walcott Tuff with overlying basalt contains abandoned dam tunnels (gated).
Abundant fragments of black obsidian on ground.