EarthCache

Great Blade Notch

A cache by bjornstad Send Message to Owner Message this owner
Hidden : 7/17/2010
In Washington, United States
Difficulty:
2.5 out of 5
Terrain:
4 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:


Cache Elevation: 1,700 ft

General Directions: On SR 17, 9 miles north of Soap Lake or 10 miles south of Dry Falls Visitor's Center, turn east onto short access road to Lake Lenore Caves within the Lake Lenore State Wildlife Management Area. Follow well-marked trail to Lake Lenore Caves and beyond.

See You Tube for video footage taken from the Great Blade by Tom Foster (hugefloods.com).

Great Blade Notch

"...there were a few double falls each member of which receded at approximately the same rate, so that the island in mid-channel became very much elongated, like a great blade, as the falls receded and the canyons lengthened." J Harlen Bretz (1928)

A tall, narrow basalt ridge, coined "The Great Blade" by J Harlen Bretz, parallels Lower Grand Coulee east of Lake Lenore (Figure 1). The blade is the product of Ice Age floods that repeatedly rampaged Grand Coulee as recently as 15,000 years ago. Most of the floods appear to have come from sudden outbursts from glacial Lake Missoula. During flooding the coulees on either side of the Great Blade were filled with up to 800 ft turbid water. The largest floods also overtopped the Great Blade, submerging the Earthcache site under at least another 100 ft of floodwater.

Figure 1. Like a giant snake the Great Blade winds through Lower Grand Coulee. The rock blade divides Lake Lenore (left) from East Lenore Coulee on the right. The Earthcache is located at a distinct notch along the Great Blade. Looking north.

On the west side of the blade, where Lake Lenore is located, lies the Lower Grand Coulee, which ultimately migrated 10 miles northward - all the way to Dry Falls. On the east side of the blade is the higher East Lenore Coulee, which migrated a shorter distance (~3 mi) to Dry Coulee (Figures 2 and 3).

Figure 2. Shaded-relief map showing the coulee system that developed within Lower Grand Coulee and places east. The Great Blade was formed by the simultaneous recession of two cataract canyons, one along the Lower Grand Coulee and the other within East Lenore Coulee. The recession of East Lenore Coulee stopped just before breaching the divide into Dry Coulee. During Ice Age flooding the entire area was submerged except the Waterville Plateau and the tops of High Hill and Pinto Ridge. Block arrows indicate primary flood-flow direction.

Figure 3. Topographic map of the Great Blade and surroundings. Hogback islands in Lake Lenore are flood-ravaged remnants of tilted basalt along the Coulee Monocline.

Like a gigantic rib the Great Blade is tallest and narrowest at its south end, widening to the north. The blade extends for almost four miles from where the head of East Lenore Coulee intersects Dry Coulee. In places the blade narrows to as little as 800 ft wide (Figures 3 and 4).

Figure 4. Aerial photo showing criss-crossing, giant grooves and potholes of East Lenore Coulee. The grooves and potholes were eroded into the top of the Grande Ronde basalt by powerful flood currents moving through this area. Dashed white line indicates trail to Earthcache. Block arrows indicate primary flood-flow directions.

Just east of Lake Lenore and The Great Blade is a 5.3-mile-long hanging coulee referred to here as East Lenore Coulee (Figures 2-4). Bretz called this now completely dry coulee "the synclinal valley". A two-tiered recessional cataract canyon, it may have started out as a stream valley that followed the trough of a geologic syncline (downwarped trough). Later, Ice Age floods naturally followed this ancient valley carving scabland along its reach. However, the enormous volume of floodwater descending through the area simultaneously flowed along the edge of the Coulee Monocline to the west. Bent layers of basalt were more broken along the tilted monocline making the rock easier to erode. Thus, the Lake Lenore channel was deepened faster than in the East Lenore Coulee. In this way the dominant flood channel eventually shifted west into its present position (Lower Grand Coulee) by carving more into the folded, more broken rocks of the monocline. Today ancient hanging valleys, perched high above the west wall of the coulee attest to the deep scouring that occurred during multiple Ice Age floods within the Lower Grand Coulee (see Figure 7).

East Lenore Coulee is bounded on the west by the Great Blade and on the east by High Hill, where several faceted escarpments were eroded along the up-to-¾-mile-wide channel (Figures 3 and 4). Like stairsteps, two basalt tiers and cataracts occur, one each in the upper and lower ends of the coulee. The head of East Lenore Coulee ends at the divide crossing with Dry Coulee (Figure 3). During the last, or one of the last, Ice Age floods spillover into East Lenore Coulee began to breach the divide with Dry Coulee. This is apparent from the hanging channel at the head of the coulee that today is perched high above the floor of Dry Coulee (Figure 3).

Figure 5. A giant rib of eroded basalt, the Great Blade, separates Lake Lenore and Lower Grand Coulee on the right from East Lenore Coulee on the left. Earthcache is located at left (east) end of narrow notch in the Great Blade, visible in the foreground.

An elevated, rock bench of Grande Ronde basalt lies along either side of the lower East Lenore Coulee. The rock bench is pockmarked with dozens of circular, deep potholes (Figures 1 and 4). Another interesting feature is a group of giant, intersecting grooves that lies midway within the East Lenore Coulee (Figures 4 and 6). The grooves criss cross each other, intersecting at oblique angles. One direction generally aligns with the flood-flow direction; grooves going in other directions appear to reflect erosion along curvilinear cracks (fractures) within the basalt. These curving fractures may be large-scale cooling features that formed as the basalt solidified from molten lava, or perhaps are related to more-regional tectonic forces applied to the basalt after the lava cooled. Ice age floodwaters rushing over these fractures etched out the weaker and more easily eroded rock along the fracture planes.The deep notch in the Great Blade where this Earthcache is located lies along one of these flood-etched curvilinear features (Figure 4).

Figure 6. Criss-crossing grooves in East Lenore Coulee viewed from the Great Blade.

From the Earthcache (Figure 7), at the east side of the Great Blade notch is a breathtaking view into East Lenore Coulee. The Earthcache lies at an elevation of ~1,700 ft, one hundred feet or more below the height of the largest floods. The Great Blade and access to the notch is owned and managed by Sun Lakes State Park. The best way to access the notch and Earthcache are to hike past the Lake Lenore Caves, then circle back to the north after reaching the giant, overhanging pothole (N47deg 30.738min, W119 deg 29.880min) , and rock bench above the caves. From N47deg 30.818min, W119 deg 29.633min is a moderately steep, 300-ft scramble up to the Great Blade notch. The Earthcache is located at viewpoint at the far southeast end of the notch (Figure 5).

Figure 7. View from Earthcache. At eye level are eroded remnants of the volcanic, 15-million-year-old columnar basalt of the Roza Member, capped by the Priest Rapids Member– the youngest basalt flow in this region. During the largest floods even the top of the Blade was completely awash in floodwaters. Several hanging valleys are visible in the background; these were carved out by ancient streams draining off the west side of the Coulee Monocline, which was later removed by Ice Age floods, leaving the tributary valleys high and dry. Looking southwest.

To receive credit for finding this Earthcache answer the following question: "How many hanging valleys are visible along the west wall of Grand Coulee from the Earthcache site?" Email the answer to bjorn99352@yahoo.com.

Additional Hints (Decrypt)

Npprff gur Terng Oynqr Abgpu ivn genvy gb Ynxr Yraber Pnirf

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)



 

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Current Time:
Last Updated: on 5/8/2017 6:46:21 AM Pacific Daylight Time (1:46 PM GMT)
Rendered From:Unknown
Coordinates are in the WGS84 datum