Fracture Fillings at Weaver Pit, Walla Walla Valley, WA

College students with access to quality information on the geology exposed near their schools will cause Geology professors to raise their game. That's a good thing for all involved. Specific information on classic outcrops and mapping areas should not be hidden away in faculty file cabinets (or memories) and sprung on students in the field. Even if guidebooks, articles, departmental photo galleries, and websites only provide the basics, its better than nothing. By simply providing 2-3 web links that students can check out before class, the knowledge floor is raised significantly. People kind of know what's going on before they get there, before anyone steps out of the van. And discussions that occur at the outcrop have a much better chance of moving beyond the easy stuff. Ideas for new research and student-scaled projects will enter the conversation (another good thing). Students can and should search for answers themselves in supplementary materials, instead of having to rely solely on their professor for help. Front-load field trips to improve in-field learning.


Fractures in the basalt bedrock exposed at Weaver Pit, a quarry located along the base of the Horse Heaven Hills near Walla Walla, contain a variety of fillings.

Yellow Cemented Fills. Yellow colored silica (and some CaCO3) fills deep vertical fractures that cut the entire high wall, a distance greater than 6 meters.

Green Opal. Opal fills vertical joints in the basalt.

White Calcium Carbonate Stringers. CaCO3 fills thin horizontal fractures formed in the exposed and weathered surface of the uppermost basalt flow at the quarry. Calcium carbonate also fills a somewhat wider vertical fracture located just left of the hoe. A half meter of Holocene (modern) loess overlies the Miocene basalt. How long was the basalt surface exposed? Is this a Pleistocene surface or a Pliocene one?

Sideways Opal. Green opal (amorphous silica precipitated from hot fluids) occurs along subhorizontal fractures, around pillows with palogonite, and along contacts between flows in the basalt. Can we tell how warm the fluid was from a regional geothermal temperature map?

Weathered Fracture. A thin fracture with no filling forms a micro-crack pathway for surface water infiltration and accelerated weathering. Though the fractures are barely visible, the weathered zone measures more than 10cm wide.

Sheeted Clastic Dikes. Vertically-sheeted silt-sand dikes, composed of sediment sourced in the overlying Touchet Beds, exploit vertical tension fractures near the hinge of a gentle fold in the basalt. They pinch downward and penetrate several meters into the basalt bedrock. Thin sections?

Gouge Zone & Clastic Dike. Some fractures exhibit a small amount of offset (shear), thus qualify as faults (not joints). Broken chunks of rock and rock powder form a gouge zone that has weathered (orange, oxidized, some clay). A thin, gray silt-filled clastic dike, seen at right originates in the overlying Touchet Beds and intrudes the gouge zone, post-dating it. The gouge provided a lower-resistance pathway for the both weathering (mostly due to infiltrating rain water) and the clastic dike. Why do the widths of the gouge zone vs. the width of the silty dike differ so much? Is the gouge show evidence of a single opening or multiple narrow openings?

Mixed Media. The vertical fracture contains some broken basalt (gouge), Touchet Bed sediment, pedogenic CaCO3 cement, and a plug of loess (silt diamict). Note how the CaCO3 overprints the rubbly, subhorizontal surface of the basalt on either side of the fracture. The cement (caliche), formed by soil processes, is part of a weak dryland soil profile developed into the weathered bedrock surface. Also note the thin stringers of carbonate that extend horizontally away from the fracture and the chips of caliche contained in the plug of tan sediment that just slumps into the top of the fracture. Is that gravel on the right? How might we tell how old the fracture is? A lot is going on in this one little fracture.

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