I try to keep the geologic formations straight when venturing on the Colorado Plateau, but the smaller stuff is just as fascinating. Geology nuances that were not as noticed as much as on previous visits. I would note that my non geologists travelling companions generally could care less about the names of formations, but often appreciate an explanation of why things look the way they do.
One feature is the concept of a hard resistant rock formation forming steep spectacular cliffs with softer rock below protected by the cap rock.
The cap rocks are the very upper part of the mesa which are underlain by Shinarump Conglomerate of the Chinle Formation. The huge red cliffs are the De Chelly Sandstone of the Cutler Group. Below the sandstone is the softer Organ Rock Shale also of the Cutler Group.
The cap rock of Shinarump is not very thick at these mesas near Monument Valley, but is the critical protective layer. While the De Chelly Sandstone forms the scenic cliffs and is clearly capable of standing as steep vertical cliffs for very long periods, once the cliff face peals away and tumbles down the slope, the increased surface exposure makes quick work of the sandstone. Hence, there is very little talus apron of De Dhelly Sandstone at the base of these high cliffs.
Note the lack of talus below the cliff wall where sandstone blocks had previously fallen out of the cliff face. More recent rock falls to the left and right have not yet been turned into sand.
If not for the ready breakdown of the De Chelly Sandstone blocks from angular boulders to sand, the monuments of Monument Valley would not exist but instead would be mounds of talus with a low cliff near the top at most.
Note the near lack of talus blocks at the base of this monument.
The lack of talus also allows for ready viewing of the underlying Organ Rock Shale.
While the hard De Chelly Sandstone is readily turned to sand when surface area is increased post rock fall from the cliffs, some rock types are very resistant to erosion even with large surface exposure. The silica rich petrified forest logs in Petrified Forest National Park weather out of the soft mudstone of the Petrified Forest Member of the Chinle Formation, and in places form a resistant veneer of silica rich wood on the surface or as scattered logs laying on the ground surface after the surrounding mudstone as been completely eroded away.
Note log encased in mudstone on cliff face
Most of the red boulders below the cliffs are blocks and logs of petrified wood.
Some hard deadlines, too much field time, disconnected from the internet and a vacation has limited posting much on the Washington landscapes. Travel and work out of state though is an opportunity to gain new perspectives.
Earlier this winter I noted that junipers are not a common tree in Washington State (juniper-dunes-wilderness), but elsewhere in the western United States junipers are taking over large tracts of the land.
I noted an odd shape shaped forest feature while flying over the Mountain Home Range in Utah.
The view of this one square mile section is of Utah State owned land. Utah did some juniper clearing on the land to improve grazing but leaving some trees in a shape that stood out when viewed from above. The clearing was done about three years ago. The surrounding land is managed by the BLM with another Utah section just over the ridge. It appears that Utah is being a bit more aggressive on Juniper management, but some of the BLM land has also been treated.
A personal note. A year and a half ago Lisa and I bought a property on Samish Island that had a shop building that was readily converted into an art studio. Lisa had out grown the tiny studio in the back of our home in Bellingham. She can now work on multiple large canvasses as well as smaller ones.
Some of her recent work is currently on display at Smith and Vallee Gallery in Edison, Washington. The paintings are of places in Washington State that we live near or frequent in our travels. A visit to Smith and Vallee will give you a good flavor of Washington landscape paintings.
Dan McShane is an engineering geologist with Stratum Group, a geology and environmental consulting company based in Bellingham, Washington. Dan has been reading Washington State landscapes since driving across the Horse Heaven Hills with his father and brother in 1970. Dan's wife has started painting Washington landscapes. The intent of this blog is to help all Washington travelers better understand the landscapes we see and share field observations.