"The monument is equally rich in geologic history, with dramatic landscapes that reveal the creative forces of tectonic, volcanic, and erosive power . . . the monument contains significant geological and paleontological objects. The late-Miocene to late-Pliocene Ringold Formation, known as the White Bluffs, was formed from river and lake sediments deposited by the ancestral Columbia River and its tributaries. These striking cliffs form the eastern bank of the Columbia for nearly half of the length of the Reach, and are significant for the mammalian fossils that they contain. Fossil remains from rhinoceros, camel, and mastodon, among others, have been found within these bluffs. . . . The Hanford Dune Field, located on the western shore of the Columbia in the southeastern part of the monument, is also of geologic significance. This active area of migrating barchan dunes and partially stabilized transverse dunes rises 10 to 16 feet above the ground, creating sandy habitats ranging from 2 to several hundred acres in size." — President William Clinton, June 9, 2000, in the Presidential Proclamation creating the Hanford Reach National Monument.

Lava Flows

Much of the bedrock of the Columbia Basin was formed by lava flows between 15 and 17 million years ago, a blink of an eye in geologic terms.

During the late Miocene and Pliocene epochs, one of the largest flood basalts ever to appear on the Earth's surface engulfed about 63,000 square miles of the Pacific Northwest, spreading over Idaho, Oregon and Washington. Eruptions were most vigorous from 17 to 14 million years ago, when over 99% of the basalt was released, although flows continued until six million years ago. Perhaps "vigorous" isn't the right word since, for the most part, the lava flowed out of cracks in the ground, rather than erupting with the explosiveness of Hollywood movies. Flows were much like you see in Hawaii today, which are still dramatic, instead of the cataclysmic eruption of a volcano like Mt. Hood.

One result of the area being formed by flood lavas are the large columnar basalts that can be found on the Monument. When thick lava cools slowly, contractional joints or fractures form. While the lava can shrink in the vertical dimension without fracturing, it can't easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns. These structures are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed. The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (½-inch diameter) columns, while slow cooling is more likely to produce large columns.

Under the Monument, these basaltic lava deposits (called the Columbia River Basalt Group) are over 13,000 feet thick. Although the Columbia River Basalt Group is one of the smallest flood-basalt areas, the individual lava flows that comprise the area are some of the largest found on Earth. Within the area covered by the Columbia River Basalt Group are two distinct subbasins, each having its own geologic character—the Columbia Basin and Pasco Basin.

Columbia Basin

This basin encloses the Columbia River Basalt Group, which is surrounded by the Cascade Mountains to the west, Rocky Mountains to the northeast, and Blue Mountains to the southeast. The Columbia Basin lies mostly in southeastern Washington but does extend into western Idaho and northern Oregon.

Pasco Basin

The Pasco Basin is a depression in the lower part of the Columbia Basin. Geographically, the ridges surrounding the Monument and vicinity define this smaller basin; the basin is bounded by the Saddle Mountains to the north, Naneum Ridge to the west, the Rattlesnake Hills to the south, and the Palouse Slope to the east—generally the area north of where the Snake River flows into the Columbia River. These ridges surrounding the Pasco Basin are the result of tectonic forces. The Pasco Basin is filled with Ringold sediment from the ancestral Columbia River and sediment left by the Missoula Floods. The Hanford Site comprises the southern portion of the Pasco Basin, occupying over 586 square miles.

Ice Age Floods

Floods that equaled half the volume of Lake Michigan but only lasting a week, tearing across the landscape at 80 miles per hour, transforming every thing in its path. Stripping some places to bedrock, while at the same time depositing the rich soils of the Willamette Valley. Nothing has shaped the Columbia Basin like the Ice Age Floods.

The last major geologic event to shape the Monument was the Ice Age Floods. During the freezes and thaws that occurred in the last Ice Age—the Wisconsin—ice dams formed and failed across several river systems in the Northwest. The largest and most frequent floods—and the ones with the greatest impact to the interior Columbia Basin—came from glacial Lake Missoula in northwestern Montana, where ice dams across the Clark Fork River formed and failed many times, each time releasing a wall of water that surged southwest through the Columbia Basin, inundating most of the Monument several hundred feet deep. However, smaller floods may have escaped down-valley from glacial lakes Clark and Columbia along the northern margin of the Columbia Basin or down the Snake River from glacial Lake Bonneville.

The Ice Age Floods began as early as one million years ago, with the most recent occurring around 13,000-15,000 years ago. The Ice Age Floods inundated the Monument dozens—if not hundreds—of times. The floods affected the landscape in different ways. As the water moved across eastern Washington, it eroded the basalt, forming channels of barren rocky land referred to as the channeled scabland. At other localities, such as away from the main flood channels, the water deposited massive bars of sand and gravel in only a few days. Where the water ponded behind obstacles such as Wallula Gap, it left behind deposits of sand and silt known as the Touchet Beds.

When the floodwaters entered the Pasco Basin, they quickly became impounded behind solid basalt in the Wallula Gap, which was too restrictive for the volume of water involved. This temporary, reoccurring hydraulically dammed lake is known as Lake Lewis. Lake Lewis is estimated to have had a surface area of approximately 4,500 square miles and to have reached a depth of about 900 feet. In the Monument, the elevation of the lake level at times rose to 1,200 feet above sea level, which corresponds to the 1200 Foot Road near the toe of Rattlesnake Mountain. The reoccurring lake is estimated to have lasted only a week or less.

The floods are of such national interest that in 2009 Congress created the Ice Age Floods National Geologic Trail. The National Park Service is currently in the process of 'creating' the trail, which will link together existing local, state and federal sites, facilities and attractions into a "partnership" park that can be driven from point to point to tell the story of the floods and their impact on the Pacific Northwest. To find out more, please visit www.nps.gov/iceagefloods/.

Since the end of the Missoula Floods, winds have reworked the deposits of sand and silt left behind, shifting them into dune sands in the lower elevations and loess (windblown silt) around the margins of the Pasco Basin. Anchoring vegetation has stabilized many sand dunes. Where human activity has disturbed this vegetation, dunes have been reactivated. More recently, dunes have been reactivated by the removal of vegetation resulting from fires.