Traveling Southwest off of Highway 12 towards the Washington-Oregon border you will come across one of the most peculiar sights of the Columbia Plateau, the Twin Sisters rock formation. Looking at the rock formation many questions may arise, such as what caused this rock formation? Why is there nothing similar around? And why are these peculiar towers in a canyon? All these questions can be answered with clues from the surrounding areas as well as the extensive research from great geologists. First of all, let’s start out with what type of rock the Twin Sisters is made. From what has been researched “the Columbia River Basalt Group (CRBG) cover an area of 210,000 km2”, from the top of the Palouse to the Pacific Ocean down to the southeastern border of California and Oregon (Camp et. al., 2017). It is estimated that the CRBG is one of the youngest flood-basalt areas on Earth with multiple basalt floods dating in around 17-6 million years old (Camp et. al., 2017). Right in the middle of the Columbia Basin is the thickest part of the basalt, which is estimated to be about 4km thick (about 2 miles), this is right around the Wallula Gap where the Twin Sisters resides (Camp et. al., 2017).
So why do the basaltic towers look so out of place in the Wallula Gap? It is a result of multiple floods from Glacial Lake Missoula eroding away the weak rock. The floods were created from the Cordilleran Ice Sheet coming down from Canada creating an ice dam that backed up a number of river drainages including the Clark Fork river near the Idaho-Montana border (Bjornstand, 2006). The result was a “lake that was up to 2,000 feet deep and covered 3,000 square miles of western Montana”, (Bjornstad, 2006). After so much pressure from the shear amount of weight the water began to deteriorate the dam and eventually broke it. This meant that “the entire lake would suddenly let loose consisting of up to 500 cubic miles of bashing, grinding, roaring water that raced through Idaho, across the Channeled Scabland of eastern Washington and down the Columbia River all the way to the Pacific Ocean”, (Bjornstad, 2006). This huge mass of water at a very high velocity tore through eastern Washington, carving out some of the most peculiar landforms. The floods went down three central paths on their way to the Pasco Basin, but once it reached the Wallula Gap there was so much water that it couldn’t all flow through it at one time. The water backed up and created a temporary lake, Lake Lewis, which covered The Pasco Basin in 900 feet of water (Bjornstad, 2006). The Twin Sisters landform was “a product of the floods, eroded into their present form beneath up to 600 feet of water during the largest floods” (Bjornstad, 2006). Looking at the rock formation further “The Two Sisters occurs at the intersection of two tectonic fractures that criss-cross the area. It appears floodwaters eroded weaker rock along the fractures, while the wedge of the stronger rock adjacent to the fractures, the Two sisters, survived the floods wrath”, (Bjornstad, B 2006).
Not only is Twin Sisters a fascinating geological spot in Southeastern Washington but it is also social significance to the area and its history. Long ago the Columbia Plateau was home to the Cayuse Indians, “the people who became known as the Cayuse were given that name by French-Candian fur traders, who called them Calloux, meaning “Rock People,” because of the rocky nature of the parts of their homeland” (Tate 2013). This is probably because of the basaltic structures that are exposed all throughout Eastern Washington. As well as the many erratic’s, due to the Great Missoula floods, that are home to different areas of Eastern Washington.
The Cayuse “had a vast homeland of over six million acres in what is now Washington and Oregon”, (Tate 2013). They were not a very big tribe, but they had great influence in their location. The Cayuse Tribe is most infamously known for the killing of Marcus and Narcissa Whitman, also known as, the Whitman Massacre. Which took place on the Whitman Mission just outside of Touchet, Wa (Tate 2013).
The Twin Sisters rock formation has stood the test of time from the erosional agents of the semi-arid climate of the Columbia Plateau. Having been formed from the Missoula Floods, it has a great story to tell being 13,000 years old. The Cayuse tribe has a legend about the basaltic towers over the Columbia River, taken from a sign just below the rocks,
The large basalt pillars in front of you are actually two Cayuse Indian Sisters. Coyote, a spiritual hero of many Indian Legends, fell in love with the three sisters who were building a trap in the river to catch some salmon. Always the trickster, coyote watched them and at night he would destroy their work. The sisters rebuilt the trap daily but Coyote would destroy it each time. One morning Coyote saw the sisters crying, they were starving for fish, Coyote promised to build them a trap if they would become his wives. The sisters consented and he kept his promise. For many years Coyote lived happily with the sisters but after a while, he became jealous of them. Using his supernatural powers, Coyote changed two of his wives into basalt pillars. The third wife he turned into a cave downstream. He then turned into a rock that so that he could watch over them forever.
Today, the rocks are apart of the beautiful breathtaking landscape of the Wallula Gap. You can hike up to the base of the basalt pillars and enjoy the view of the mighty Columbia River. The Wallula Gap and the Twin Sisters rock formation are some of the best examples of just how destructive the Great Missoula Floods were to the Eastern and Southeastern parts of Washington State.