Special Geologic Features

White Bluffs

Apart from the White Bluffs, several unique or special geologic features are found on the Monument.

The White Bluffs have already been discussed, but a bit more is worth mentioning. The spectacular White Bluffs of the Columbia River line the river for about 20 miles north from Richland. The bluffs—900 feet tall at their highest point—are composed of gravel, sand and silt deposited by ancient lakes, the Columbia River and its tributaries. The bluffs are a stacked column of many different depositional periods, taking millions of years to accumulate. In the intervening years, the Columbia River has cut through this sediment, exposing the bluffs to the enjoyment of all.

Less known and visible, but equally as interesting, world-renowned bedrock basalt flows have ridge features (known as wrinkle ridges) analogous to those on Mars and possibly other portions of the solar system.

Sand dunes are located in various parts of the Monument, the result of reworked Pleistocene flood deposits being driven by high winds in the Columbia Basin. The dunes on the Monument are predominantly parabolic, although barchan and transverse dunes appear. The most notable dune field, located in the southeast corner of the Monument, was specifically included as part of the Monument in order to protect this unique geological phenomenon. These active, primarily unvegetated dunes range from 10 to 16 feet high, can cover several hundred acres, and migrate in an east to northeast direction at a rate of eight to fifteen feet per year. Various other dunes can be identified within the Monument landscape. The top of the White Bluffs have a field with both migrating dunes on the bluff edge and stabilized dunes to the east. Smaller dune areas are located within the ALE. Movement and stability varies depending on natural factors (fire, wind, vegetation) and human intervention (surface disturbance).

In addition to leaving behind sand to form dunes, the Missoula Floods contributed several other geologic features to the Monument. Glacial erratics are non-indigenous rocks, such as granitics, gneiss, quartzite, argillite and schist, carried on ice rafts by the raging flood waters from as far away as Montana and Idaho. These rocks, often reaching boulder size, are scattered around the Monument, up to the highest lake levels reached, about 1,200 feet on the flanks of Rattlesnake Mountain. Other unique features connected to the floods are bergmounds, giant ripple marks, and gravel bars. All of these features are considered slack water deposits that occur when turbulent water action subsides. The southwestern end of the Monument contains such topographic high points because the northern base of Rattlesnake Mountain was the periphery of the lake. The ripples and gravel bars are formed primarily of sand and gravel carried along by the sheer power of the flood waters. As the water was slowed by natural features, such as the Wallula Gap, or the floods naturally receded, the scouring power of the water was reduced, and the sands and gravels were deposited out. Like erratics, bergmounds were left when icebergs rafted lithic material from other areas. Bergmounds are typically composed of small gravels of the same materials as erratics but are found at slightly lower elevations, usually below 1,000 feet. It is believed that bergmounds were formed when larger icebergs grounded themselves at the shallow edges of the lake as the flood waters withdrew. Being larger and deeper, they hit bottom sooner than those carrying the erratic boulders. Once grounded, the ice melted, depositing the iceberg’s load of sand and gravel in place.