Geomorphology of the northern Thorp Region

The small hamlet of Thorp, Washington, lies about 14 minutes to the west of Ellensburg as the cwu geology van drives. It is located in the Yakima River Valley, the namesake of which meanders it's way north of Thorp slowly making it's way southeast. 

Today I ventured out on a field trip with my Geomorphology class to observe some of the Geomorphological features of the area. What is Geomorphology? Just break it up! Geo=Earth, Morph=Change, and Ology=To Study. So putting it all together, it's the study of landforms created through geologic processes. 

The first feature we observed was the most interesting to me personally. Almost 2.8 miles due north and a little west of Exit 101 on Interstate-90, is a large hillside comprising of ancient Yakima River Sediments. At this point, 1.5 to 5 thousand years ago, the hillside failed. Possibly driven by weak clays or saturated soils, a massive rotational-slump type landslide broke loose. The resulting earth flow at the foot of the slide shoved the Yakima River south by several hundred feet. 

Looking across the large landslide north of Thorp. Photo by Micah K. 

From an overlook at the western edge of the landslide, my Geomorphology class looked across this landslide. The displaced material had taken on a hummocky appearance, with the blocks tilted slightly towards the failure headwall. The size of this thing was boggling, even while small in comparison with the landslide that, for example, decapitated Mount Saint Helens in 1980 with catastrophic results. Even the several houses that were built on top of the landslide deposits seemed small in comparison. It was truly amazing.

Northwest of Thorp, Interstate 90, climbs up another large slope and across the wide, rolling expanse of basalt boulders and sagebrush known as the Thorp Prairie. It's strangely empty up on this expanse, only being occupied by several old and dilapidated windmills, and crisscrossed by irrigation canals. The reason for this emptiness being most in part for it's geology. Thorp Prarie sits on a massive terminal moraine formed many thousands of years ago when a large glacier flowed down the Yakima River Valley. The heart of this moraine is filled with rocky glacial garbage, scraped out from the mountains and dumped here as the glacier retreated, too rocky to farm on, and not very suitable for irrigation, this, it has remained relatively untouched. 

As the Yakima River carves it's way through this moraine to the north, over thousands of years it has created several large flood terraces visible on the flank of the moraine and elsewhere in the valley walls. These large flat expanses are sometimes built directly on top of Columbia River Basalt, which is exposed in the deepest parts of the canyon by the river. And as luck would have it, I'm going to see a important portion of the Columbia River Basalt tomorrow. So...to be continued......

Erosional Remnant of the Columbia River Basalt. Photo by Micah K.