Wallula Gap is a large break of the Basalt anticlines through which the Columbia River flows, located just south of the confluence of the Walla Walla and Columbia rivers. It has been recognized as a National Natural Landmark by the National Park Service as a site that provides an important illustration of the geological history of the United States.
Ancient Lave Flows
Millions of years ago, basalt flows flooded the region with extremely fluid lava. This spread in all directions throughout southeastern Washington, eastern Oregon, and southern Idaho, from long fissures in the Earth's crust. The results were broad fields of gently sloping lava that spread widely over great distances. These lava flows have been exposed by erosion near the Snake River in Idaho, and the Columbia River in Washington and Oregon (with extensive displays of columnar basalt), and measure almost 2 km (1.24 mi) thick.
The basalts flows visible in the Wallula Gap were laid down about 17 million years ago, during the Miocene Epoch. They are part of the massive fissure flood basalts of the Columbia River Basalt Group. During this time, the fissure floor, the southern and western Columbia Plateau (which included the current location of the Wallula Gap), began to fold. The bending (or shear) of these flows is visible in the layers of basalt exposed in the walls of the gap. Creating an almost stair step effect.
How Wallula Gap was Created
Early on, when the lava flows were folding, the ridge crest at Wallula Gap must have been one of the lowest points. The Salmon-Clearwater River began flowing across the ridge at this low point, creating a channel. As the lava flows continued and the ridge continued to fold upward, erosion from the river continued to cut downward and Wallula Gap was born. Until around 10 million years ago, the Salmon-Clearwater River was the only source of water moving through the gap. Around 6 million years ago, as the flood plain in the Columbia Basin continued to tilt, the Columbia River was forced eastward. It was around this time that it joined with the Salmon-Clearwater River. Around 3.5 million years later, the Snake River was forced northward by the uplift of the Blue Mountains and joined with the Salmon-Clearwater River near the Idaho-Oregon border.
Lake Missoula Floods
Around the 14th millennium BC, The Wallula Gap was widened, by the Salmon, Snake, and Columbia rivers combined with the glacial flood waters that poured across the Channeled Scablands during the Ice Age Floods. The Wallula Gap slowed the flow such that less than 1/5 of the 800 km³ (192 mi³) of water per day entering could be discharged as it passed downstream into Lake Condon. That's 193 cubic miles of water! Due to the constraint of the water, the floods filled the Pasco Basin and formed the short lived Lake Lewis. The incredible volumes of flood water passing through the gap contributed substantially to the erosion of the gap. This is still visible today when you look at the shear walls and by scab-features such as "Twin Sisters".
During the Missoula Floods, due to the narrow opening of the Walla Gap, water backed up in the Pasco Basin. The floods were so large, the water couldn't discharge through the gap quickly enough. Even though it is 2 km (1.2 mi) wide! At their height, flood waters reached elevations of about 380 meters or 1250 feet. The amount of pressure created or hydraulic head (elevation drop) required to drive the flood waters through the gap, combined with the elevated levels downstream of the gap, resulted in a head of about 250 meters or 820 feet. That's 820 feet of water exiting South, out of Wallula Gap. The flow, at its peak, is estimated to have been about 10 million cubic meters or over 350 million cubic feet per second.
Source: Wikipedia
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Wallula Gap is unique, in that, as you travel Southbound on Hwy 12, the gap appears to close before your very eyes. Almost as if a pair of sheer's blades are closing before you. There is no need to stop anywhere along the highway. In fact, if you stop, the phenomenon will fail to appear. Use caution and don't stare at the gap while driving, just an occasional glance out of your peripheral vision is all that's required. You must also be heading Southbound, if you are traveling North, exit at the town of Wallula and return South towards Wallula Junction. It's best to have a partner when completing this Earth Cache, but I did it safely alone.
Traveling the speed limit (60mph), start counting when you reach the posted coordinates. 1-one thousand, 2-one thousand and so on. For safeties sake, don't look at your phone or GPSr when approaching, simply start counting when you pass the large, brown informational sign along the highway. Continue counting as you travel South, until the gap has completely closed. You'll be surprised by the time it takes and the strange sense of such a large area closing before your eyes. The reason for this phenomenon, is due to how the Basalt layers formed and were then carved out by the torrents of water that followed. To get credit, tell us how long it took the gap to completely close. Thank you and we hope you enjoy.