Argillite Boulder – Rattlesnake Mountain, Washington
This image depicts an interesting perspective of time. The rock in the foreground was deposited here on Rattlesnake Mountain around 15,000 years ago. The Rattlesnake Mountain and the Horse Heaven Hills in the background were created as part of the Yakima Fold formation about 1.5 million years ago. The bedrock of this area is basalt from basalt flows through eastern Washington from 6-15 million years ago. The argillite boulder in the foreground is metamorphic rock from western Montana formed 1.5 billion years ago.
So here I was, sitting on a rock created before life on earth existed, brought here by a humongous flood around 15 thousand years ago, deposited on mountains uplifted 1.5 million years ago, created by a series of gigantic lava flows about 2 miles thick 6-15 million years ago. And I think I am old at 67 years. As many of my Whizzy friends would say, “It’s a thinker.”
As glaciers moved southward during the Ice Ages, they scoured the terrain picking up rock debris. During the Missoula Floods, parts of the glaciers would break off forming ice bergs. These were carried down through the Eastern Washington scablands into the Pasco basin. As Lake Lewis formed, many of the ice bergs floated to the edges of the lake. As the lake emptied, several of these ice bergs were left stranded on the surrounding ridges. They melted leaving mounds of accumulated rocks, gravel, and sand. These are “bergmounds”.
Most bergmounds are found in the Pasco Basin at elevations of 600 – 850 feet. They are 20 – 35 feet higher than the surrounding terrain. The bergmound pictured above is on a plateau of eastern Rattlesnake mountain above Richland, WA. The bergmounds are somewhat inconspicuous unless, one is looking for them.
Granite Erratic – Rattlesnake Mountain, Washington
The Missoula Floods carried large icebergs from the glaciers that dammed Lake Missoula or from the Okanagan lobe glacier that dammed Lake Columbia along with them as they made their way to the Pacific. As the icebergs melted or became “stranded” against ridges that formed Lake Lewis, they dropped the rocks that the glaciers picked up as they scoured their paths southward. Granite is present in Montana as well as northern Washington. But it is not present in central Washington. The origin of this single granite erratic on Rattlesnake Mountain is therefore not definite. It could have come from either Montana or northern Washington.
This chunk of granite is approximately 6′ in length. It is located at about 800 feet elevation (my estimate).
This image was taken looking north through Sentinel Gap. This gap cuts through the Saddle Mountains which separate the Pasco and Othello basins in Eastern Washington. The Columbia River runs through this gap on its way to the Oregon/Washington border. At the time of the Missoula Floods, the water level going through the gap reached the top of the left ridge, flowing into Lake Lewis which covered the Pasco basin.
The erratics in the foreground most likely are from the gap, carved away by the raging wall of water that flowed through it.
The Central Washington Basin is covered with layers of basalt, totaling as much as 2 miles thick, created by huge lava flows 8-15 million years ago. The nearest source of granite rock is several hundred miles away. This photo was taken near the Hanford site north of Richland. The granite rock was about 1/4 mile from the road. My lens combination was 560mm. For reference, the actual height of the rock (above ground) is about 12 feet. It is a big granite rock. How did it get here???
The current theory is that it was brought here as part of the ice dam that broke loose during one of the Missoula floods around 15,000 years ago. As the ice melted, embedded rocks were released. The Pasco basin and its surround hills have many such erratics.