McLaughlin Canyon geology begins where most eastern Washington geology ends. Just south of Tonasket, the canyon slices through a geological world most visitors never see. Instead of the mile-thick, dark lava flows that blanket the region to the south, the canyon walls here rise 400 feet in gleaming, banded gneiss. These rocks are not young, nor volcanic. They are the exposed roots of an ancient mountain range, formed deep in the Earth's crust and exhumed by tectonic forces nearly 50 million years ago. For scientists, cavers, climbers, and anyone with a sharp eye for stone, this canyon is a window into the Eocene past and the processes that built the Pacific Northwest.
Tonasket Gneiss: The Bedrock of McLaughlin Canyon
The core of McLaughlin Canyon is made of Tonasket Gneiss, a coarse-grained, banded metamorphic rock. While "Tonasket Gneiss" is an informal name used by local geologists and climbers, the USGS classifies these rocks under broader units such as pre-Upper Jurassic gneiss. The gneiss originated as granite or similar igneous rock, buried miles beneath the surface during the Mesozoic era, well before dinosaurs disappeared. Under immense pressure and heat, the minerals in the granite recrystallized and aligned into alternating light and dark bands.
Field mapping by the USGS places the protolith (original rock) as pre-Upper Jurassic in age, likely over 150 million years old. The canyon's cliffs expose these ancient roots in striking vertical faces, with quartz, feldspar, and biotite forming the bulk of the rock. In places, darker bands of amphibolite (metamorphosed mafic rock, possibly derived from ancient basalt) punctuate the lighter gneiss layers.
The visible structure records tectonic shearing, folding, and high-grade metamorphism. Climbers and hikers find the gneiss to be hard, stable, and unlike the crumbly basalt columns found elsewhere in eastern Washington.
The Okanogan Metamorphic Core Complex: Exhuming Deep Crust
McLaughlin Canyon's geology is part of the much larger Okanogan Metamorphic Core Complex, one of several "core complexes" scattered along the North American Cordillera. These structures record a period in the Eocene (about 48-51 million years ago) when the thickened crust of the Pacific Northwest began to extend and collapse. Overlying rocks slid westward on low-angle detachment faults, while deep crustal rocks rose toward the surface in giant domes.
The Okanogan dome, which includes the Tonasket Gneiss, is a classic example. As the detachment fault (the Okanogan detachment) enabled overlying rocks to slide away, the hot, ductile gneiss below uplifted and cooled, eventually breaking through to daylight. This process exposed rocks that had once been buried 20 kilometers or more below the surface.
Field evidence for this tectonic unroofing includes mylonite zones, mineral lineations, and the presence of both high-temperature and lower-temperature metamorphic minerals. Mylonites are fine-grained, foliated rocks created by intense shearing along the fault zone, displaying mineral stretching lineations and a distinct banded appearance. These features record the last movements of the fault system that brought the gneiss to the surface.
The Okanogan Metamorphic Core Complex stretches north into British Columbia and south toward the Columbia River. McLaughlin Canyon is one of the most accessible sites for viewing its dramatic, unweathered gneissic faces.
Fracture Caves: Underground Labyrinths in Gneiss
The southwestern slope of Tonasket Mountain, directly above the canyon, hides over 1,000 feet of interconnected fracture caves and crevasses. Unlike solution caves formed by water dissolving limestone or lava tubes created by flowing magma, these caves form along stress fractures in the hard, brittle gneiss. They are fracture caves, a relatively rare type in North America.
The process begins as tectonic forces stretch and break the rock, creating open cracks and voids. Over time, erosion, ice, and freeze-thaw action widen these fractures. Some passages are tall, narrow fissures with sunlight filtering in through slits 20 to 40 feet overhead. Others drop into cool, moist lower chambers that require short technical rappels and artificial light. The total mapped length exceeds 1,000 feet, though the network is still being explored.
These caves preserve evidence of both tectonic stress and the canyon's ongoing evolution. Unlike solution caves, fracture caves provide limited habitat for cave-adapted species, but their microclimates and unique mineral crusts attract biologists and mineralogists. The caves are wild, unstable, and demand skill and caution.
How Fracture Caves Differ from Solution and Lava Caves
Most caves in Washington fall into two categories: solution caves in limestone (like Gardner Cave near Metaline Falls) and lava tubes in basalt (like Ape Cave on Mount St. Helens). McLaughlin Canyon's caves are neither.
- Fracture caves (McLaughlin Canyon): Formed by mechanical cracking and widening of hard, non-soluble rock (gneiss). Passages are typically narrow, high, and linear.
- Solution caves: Created by acidic water dissolving limestone or marble. Features include rounded rooms, stalactites, and broad chambers.
- Lava tubes: Result from flowing lava crusting over and draining out, leaving horizontal, tube-shaped tunnels in basalt.
The Okanogan region has some of the best examples of fracture caves due to its tectonic history and the competence of the bedrock. Passages in McLaughlin Canyon often dead-end or pinch out, but the largest fissures can be explored for hundreds of feet. McLaughlin Canyon demonstrates that spectacular caves can form in environments far removed from limestone country or volcanic terrain.
Anatomy of the Canyon: Cliffs, Width, and Topography
The physical dimensions of McLaughlin Canyon match its geological significance. The canyon narrows to 40-100 feet wide in places, with vertical gneiss cliffs over 400 feet tall. At its mouth, the canyon opens to nearly 200 yards. These proportions create dramatic light and shadow, and the microclimate within the narrows differs sharply from the Ponderosa pine uplands above.
Banded gneiss walls, sometimes streaked with black amphibolite, rise in columns and slabs. Talus slopes and debris fans pile up at the base, fed by freeze-thaw cycles and occasional rockfall. The upper walls are scored with fractures, some of which serve as entrances to the cave system.
Vegetation clings to ledges, but the overall impression is one of stark, ancient stone. The geometry reflects both the resistance of the gneiss to erosion and the legacy of tectonic fracturing. Seasonal water flow continues to shape the canyon floor, but the sheer cliffs remain as evidence of the rock's durability.
Glacial Overprint: The Okanogan Lobe and the Great Terrace
The present shape of McLaughlin Canyon is partly a legacy of the last Ice Age. From about 21,000 to 12,000 years ago, the Okanogan Lobe of the Cordilleran Ice Sheet advanced southward into the Okanogan Valley. This tongue of ice was over 3,000 feet thick in places, thick enough to dam the Columbia River near Grand Coulee and create Glacial Lake Columbia.
While the glacier did not directly carve McLaughlin Canyon's cliffs, it profoundly altered the regional landscape. The ice scoured bedrock basins up to 650 meters below sea level in the valley floor and deposited thick sequences of glacial till and outwash. As the ice retreated, meltwater built the "Great Terrace," a complex of kame terraces that stretches over 200 kilometers along the Okanogan River valley sides.
McLaughlin Canyon sits above this terrace system, but the canyon's talus slopes and lower reaches include glacial debris. Glacial till, cobbles, and erratics can be found in stream beds and on slopes, mixed with younger alluvium. The Ice Age's impact is visible on the canyon's lower slopes, where rounded stones and layered gravels record glacial influence.
Columbia River Basalt: The Lava Blanket That Missed McLaughlin
The Columbia River Basalt Group (CRB) is one of the world's largest flood basalt provinces, covering more than 210,000 square kilometers across eastern Washington, eastern Oregon, and Idaho. These vast, dark lava flows were erupted during the Miocene, from about 17 to 6 million years ago, long after the formation of the Okanogan gneiss. The gneiss predates the basalt by roughly 30 million years.
CRB flows reach thicknesses of over a mile in the Columbia Plateau, forming the rolling scablands and coulees that define the region. They are fine-grained, dark gray to black, and typically form columnar joints as they cool. The CRB dominates the geology south of the Okanogan Highlands, but it never reached McLaughlin Canyon. The flows lapped up against the highlands but did not overtop them, leaving the ancient gneiss exposed.
This absence is critical for local geology. Where the CRB covers the bedrock, the underlying metamorphic and granitic rocks are hidden. In McLaughlin Canyon, by contrast, the gneiss is visible, accessible, and unaltered by the Miocene lavas.
Gneiss vs. Basalt: A Side-by-Side Comparison
| Property | Tonasket Gneiss (McLaughlin Canyon) | Columbia River Basalt (Eastern WA) |
|---|---|---|
| Age | Exhumed 48-51 Ma (Eocene); protolith pre-Jurassic | 17-6 Ma (Miocene) |
| Origin | Deep crustal granite, metamorphosed under heat/pressure | Surface eruptions of basaltic lava |
| Rock Type | Metamorphic (gneiss) | Igneous (basalt) |
| Texture | Coarse-grained, banded | Fine-grained, massive or columnar |
| Mineralogy | Quartz, feldspar, biotite, amphibole | Plagioclase, pyroxene, olivine |
| Structure | Foliated, banded, often sheared | Massive, often columnar jointed |
| At McLaughlin Canyon? | Yes | No |
| Cave Type | Fracture caves | Lava tubes (not present here) |
Amphibolite Bands: Traces of Ancient Ocean Crust
Within some exposures of the Tonasket Gneiss, especially where the banding is most pronounced, thin dark layers or lenses of amphibolite appear. Amphibolite forms when mafic rocks (originally basalt or gabbro, often oceanic in origin) are subjected to the same metamorphic conditions as the surrounding granite. The result is a tough, black rock composed mainly of hornblende and plagioclase.
Amphibolite bands in the Okanogan dome sequence offer evidence of the complex tectonic history. Their presence suggests that slices of ancient oceanic crust were caught up in the continental collision, buried, and metamorphosed alongside the granite. In the field, amphibolite bands are more resistant to weathering and often stand out in relief. The thickness, orientation, and mineralogy of these bands provide a direct link to the region's subduction past.
McLaughlin Canyon in the Broader Okanogan Landscape
McLaughlin Canyon is part of a network of exposures that reveal the deep crustal architecture of the Okanogan Highlands. The canyon's geology links north to the Shuswap Metamorphic Complex in British Columbia and south to the Kettle Dome. Each dome exposes different slices of the crust, but all share the core-complex process: deep burial, high-grade metamorphism, and rapid exhumation during the Eocene.
The Okanogan Highlands are a patchwork of granitic, metamorphic, and volcanic rocks, set apart from the basalt-covered Columbia Plateau. The region supports forests, meadows, and unique biological communities adapted to thin soils over hard bedrock. Water chemistry, soil fertility, and habitat diversity all reflect the underlying geology. The rocks are the foundation, and McLaughlin Canyon offers some of the region's best exposures.