Non Washington: Aerial Notes from Nevada

Flying into Las Vegas, Nevada is a always a great treat for a geologist with a window seat or for anyone that appreciates landscapes.

A top highlight is the classic Keystone Thrust:

The older rocks of the layered limestone rocks of the gray Cambrian-age Bonanza Formation are overlying the buff to red sandstone of the younger Jurassic-age Aztec Formation. The older rocks were thrust up and over the younger rocks approximately 70 million years ago with an estimated crustal shortening of 60 miles. There are plenty of thrust faults in Washington State, but not nearly so well exposed and easily seen.

To the west of the thrust fault, the older rocks are also folded. Again demonstrating the crustal shortening or compression that took place in this area.

Geologic hazards are also readily observed, such as flood hazards. Nevada is full of alluvial fan areas with associated flood, erosion and sediment deposition hazards.

Dam and excavation on right to capture and hold water and sediment to protect Las Vegas development on the left.

Parhump has seen rapid residential development with very little regard to flood hazards associated with alluvial fans. The approach to development appears to have been build first and then solve the flood hazards after floods take place. Parhump experienced flooding in 1997 and twice in 2003 including flood waters as deep as 3 to 4 feet (Nevada Division of Water Resources, 2013).

Efforts to assess the hazards have included detailed mapping of piedmont and playa features (House, Buck and Ramelli, 2010). The broad fan below shows a variety of ages on its surface and illustrates the fact that alluvial fans are complicated and subject to change during flood or debris flood events.

The Las Vegas River has caused flooding and erosion in Las Vegas that has led to a variety of flood projects on the river. But given the drought risk that Las Vegas lives with, water in the river is a good thing. That water is actually waste water from the waste water treatment plant. The water is simply being returned to where it was drawn from.

Flowing water in the Las Vegas River

The long drought is evident when Lake Mead came into view.

Bath tub ring around Lake Mead on the Colorado River

Nevada is well know for mining. Part of the fun of flying over the state is mine spotting. Below is a the classic gold mine operation with heap leaching to extract the gold from the ore:

Mineral Ridge (scorpio gold.com/mineral ridge)

See SF's post for some geology and other references for this area

Nevada has non metal mines as well including mountain top mining for limestone and mountain top development.

Limestone mine outside of Las Vegas for cement production

Building pads for home development in south Las Vegas 

The biggest recent alteration of the landscape of Nevada the past few years has been huge solar farms:

Distant view of solar farm 

100 Years of Good Work

Capitol Reef Visitor Center

This year marks the 100th anniversary of the National Park Service. America's National Parks are something to be proud of, and we should be proud of the great park service that has served our parks and us so well. I paid a recent visit to a few of the parks in southern Utah. While I often seek out more remote places, I really enjoyed sharing some more visited sites with others.

Delicate Arch is not a place to go for solitude. I did have the site to myself the first time I visited it. A heavy snow storm brought about that lonely venture. The arch is a remarkable feature, and this time I very much enjoyed sharing the site with other hikers. One of the many great places to enjoy as a nation.
 

Boulder Lag Beach

 

This shoreline stretch on the Salish Sea is nearly all boulders. Most of the boulders matched the local bedrock geology, but there are plenty of exotic boulders from far away places brought by the ice age glacier.

 

The source of the boulders is a glacial drift deposit along bluff above the beach.

 

Glacial drift with some stratification

Glacial drift directly deposited on bedrock

The shore has enough wave energy to removed the silt, sand and gravel from the beach leaving behind a lag of boulders. The headlands at bot ends of the beach were bedrock points. Any sand and small gravel had been transported into deep water off the points and outward from the boulder lag beach.

Lummi Cloud Cap

A bit of cool wet weather reminding us we are in the Pacific Northwest still - not California. The ridge of Lummi Peak on Lummi Island is often capped with a cloud as the air rises 2,000 feet to pass over the ridge. Nice site as I headed out to adventures elsewhere.

 

Avalanche Hazard

Avalanche shoots in British Columbia

A geologic hazard that has been added to several county geologic hazard areas designations is avalanche hazards. With potential future development of mountain valley properties associated with winter recreation, it is not a bad idea to consider this risk. Ideally the hazard will be recognized prior to zoning changes that would allow homes or development in these areas.

The potential areas impacted in Washington State are pretty minimal. I took a look at this potential hazard for one county and found no areas of potential development areas that could be impacted presuming no radical shift in snow fall amounts on the lower elevation areas of the county.

With a few limited exceptions avalanche hazards in Washington State are outside of development areas. I encountered one site that had deposits of large wood, boulders and soil that were a bit perplexing because I was not thinking about avalanche hazard. A review of historic aerials helped. Although no avalanches were reported, the on the ground indicators of a were consistent with a past avalanche out of a high meadow 3,000 above that reached the valley floor. Avalanche hazard assessment requires some consideration of extreme weather events much like landslide hazards, alluvial fan hazards and flood hazards.

Rock Fall and Root Throw

Assessing potential rock fall hazard on a slope meant taking a careful look at how the rock was weathering and joint patterns on cliff faces. Could this cliff spall off a large boulder? How much energy and momentum would a worse case rock fall have? What are the potential pathways of rock falls? What kind of material will the rocks encounter on the slope below? Will a rock fall maintain energy and speed while rolling or bouncing down the slope?

In assessing the cliff, I noted a rather loose boulder perched on the edge partially supported by a big leaf maple tree. My initial thought was that the tree had stopped a boulder from a cliff that was higher up the slope above the lower cliff area I was investigating.

In fact there was no cliff source area for the boulder. The slope above the cliff was a long flat area with absolutely no risk of rock fall. 

After a little thinking, I concluded that the precariously perched boulder got pulled up out of the ground some time in the past when a tree, now gone, (perhaps an earlier version of the big leaf maple) fell over. 

An example is this toppled tree that yanked a angular block of rock out of the ground and plopped it on the moss and brush. It is striking that such a modest looking tree has the root strength to pull out such a large rock from the bedrock below.

Seismic Retrofit

Last month I was involved in a remodel project of an old masonry constructed building. It is a very cool project and my involvement was limited to the potentially bad news part - seismic hazards and foundation bearing of the underlying soils. The solutions to the seismic and structural part are up to the structural engineer. The news we delivered regarding foundation loads and potential liquefaction made a huge difference in terms of costs. Hence, there was some anxiousness on the part of the project team. Part of the project did require steel support to upgrade the anticipated seismic/structural loads.

  

The site pictured above was not the project. I did not suggest this exterior steel support. The above retrofit project for a multi story school was apparently the most appropriate cost effective approach for the site conditions. More often seismic retrofit projects are not so visible. Perhaps a good constant educational reminder for the students at this school about geologic hazards.