Geologic Highlights: Recessional cataract canyons, plunge pools, flood bars, ancient dry lake beds; adjacent flood-swept, upland plateau covered with potholes and giant grooves. Eroded contact between ancient 50 to 100 million-year-old granite beneath 15-million-year-old Columbia River basalt.
Cache Elevation: 2,280 ft
Directions to trailhead: From SR 155, directly across from Steamboat Rock, turn east onto Northrup Rd. Pass eddy flood bar at mouth of canyon and ascend 0.6 mi to parking area and rest room at end of gravel road ("P" in Figure 1). From the trailhead it's a little over seven miles to the Earthcache and back.
Figure 1. Northrup Canyon area map. A tall ridge of granite runs down the middle of the canyon. At one time the granite was completely buried under basaltic lava flows but later exhumed by Ice Age floods. Block arrows indicate flood-flow directions.
Northrup Canyon is an idyllic, open-forested canyon where cataclysmic Ice Age floods once ate through a pile of layered basalt flows into granitic basement rock as they spilled over a side canyon into Grand Coulee (Figures 1 and 2). The steep walls of the multi-armed canyon tower up to 400 ft or more. Megafloods eroded through the basalt bedrock exhuming a once-buried ridge of granitic basement rock (50-100 million years old) that parallels the canyon. Long ago (15-16 million years ago), during the Miocene Epoch, basalt flows lapped up onto and around the eroded granite ridge eventually burying it in lava. Since that time the Ice Age floods eroded through the basalt cover locally exposing the granite rock below. Successive floods exhumed the ridge by taking advantage of the weaker, weathered rock along the contact between the two rock units. The last megafloods, derived from cataclysmic outbursts from glacial Lake Missoula, were about 15,000 years ago.
Figure 2. Aerial view of Northrup Canyon. Everything in this image was overrun by hundreds of feet of rushing water during the largest megafloods. Adjacent upland plateaus were also swept clean by the floods, which carved huge grooves and potholes into the basalt surface. Looking northwest.
Ice Age floodwaters spilled over into the Northrup Canyon both: 1) during earlier Ice Age floods before the Upper Grand Coulee cataract had retreated all the way to the head of Grand Coulee, and 2) when glacial ice of the Okanogan Lobe blocked the Upper Grand Coulee. Figure 3 shows how the flow of floodwater when glacial ice blocked the Upper Grand Coulee, forcing all the floodwater into Northrup Canyon and across the upland plateau above Grand Coulee. Northrup Canyon was probably not used during the last floods after the head of the Grand Coulee was breached; this event lowered the divide at the head of the canyon by as much as 900 ft, probably causing all subsequent floods to flow down Grand Coulee - bypassing Northrup Canyon.
Figure 3. Aerial view showing how floodwaters (block arrow) spilled over into Northrup Canyon from the Columbia Valley when Grand Coulee was blocked by the Okanogan Ice Lobe. Patch-worked farmland in lower right generally escaped erosion by megafloods. The end result was a flood path that was confined to the narrow upland plateau that parallels the Upper Grand Coulee.
Figure 4. Upper Northrup Canyon. Dashed outline is area of exposed granite. Block arrows show movement of Ice Age floodwaters that spilled over from the Columbia Valley to the north (visible at top of photo).
The trail to the Northrup Lake plungepool passes the now-abandoned Northrup Homestead (Figure 4). Along the way to the Homestead is a huge flood bar along the south side of the canyon and a couple flat-bottomed, now-dry lake beds (Figure 1). At Northrup Homestead, the canyon bifurcates into two main arms. One arm trends north terminating on the upland plateau only about ½ mile from the edge of Upper Grand Coulee (Figure 4) – a second arm branches southeast from the homestead. Both arms are cataract canyons that receded across the upland plateau during multiple megafloods. The trail to the Northrup Lake plungepool and the Earthcache lies at the head of a short side canyon that branches off the northern arm of Northrup Canyon. The canyon is an oasis in the eastern Washington desert, which supports the growth of quaking aspen and Douglas Fir - the only naturally-growing fir here in Douglas County!
Figure 5. Upper Northrup Canyon, looking north. A granite ridge, running down the middle of the canyon, was exhumed during multiple Ice Age floods. The Earthcache lies atop the cataract cliff above the Northrup Lake plungepool.
To make the final approach to the Earthcache continue along south side of Northrup Lake via an old jeep trail that traverses diagonally up the east side of the cataract. Once atop of the cataract walk north along the rim of the west-facing cliff precipice a short ways to the Earthcache. To the north, east, and south is the flood-swept upland plateau that surrounds all of Northrup Canyon (Figure 5). To the southwest is a spectacular view back down the canyon (Figure 6). During the largest megafloods water rose to near 2,500 ft elevation, about 200 ft higher than the level of the Earthcache and upland plateau.
Figure 6. Northrup Lake plungepool from cataract cliff at head of Northrup Canyon. A ridge of more flood-resistant granite runs up the middle of Northrup Canyon, dividing it into two lesser subcanyons. Multiple flows of volcanic basalt cover the upland plateau that surrounds the canyon.
Recession of the cataract and headward growth of Northrup Canyon probably occurred in multiple spurts from many floods. Northrup Lake represents the plungepool that developed as the floodwaters dropped over the receding cataract for the last time. During a future Ice Age flood, the cataract will likely continue its march northeastward toward the Columbia Valley.
To receive credit for finding this Earthcache answer the following question: What type of rock is underfoot at the Earthcache? Is it: 1) massive, light-colored granite, 2) layered black basalt, or 3) another type of rock?
Email answer to email@example.com.