United States of America · 10 Days · 15 Moments · April 2016

Dylan Ahearn

Dr. Dylan's family working geology vacation thing

16 April 2016

Last stop of the trip. Photo shoot at an abandoned concrete factory in NE Oregon. Tangentially geology related, mostly art related. Seattle here we come!

15 April 2016

Not a geology post. But still fun.

15 April 2016

Easy Geology I learned up on my geology in North Carolina where the rocks are buried under thick vegetation and soils (see second photo). When they are exposed it looks like they have been stirred with a stick. Old metamorphic rocks with long histories of deposition, burial, exposure, reburial, metamorphism, exposure, and faulting (first photo). It hurts the brain to try and untangle the history of such places. Out here things are more simple. On this trip we have driven through the "Grand Staircase" (see photo from my handy guide book). The sedimentary rocks are generally flat lying and well exposed which makes mapping relatively easy. Here is one more photo of another layer in the cake, this time deeper and older than the rocks of Arches, Bryce, and Zion. The rocks which make up the walls of the Grand Canyon are also exposed here outside of Vegas. The Kaibab limestone and Cedar Mesa sandstone. No so easy is the 9 hour drive we have ahead of us. Boise here we come!
We saw at Zion how stream erosion can be a powerful force shaping the landscape. We have seen many streams on this trip, but here are examples of two types. The first is a braided channel located in the Claron Formation (near Bryce). Remember the Claron Formation has a lot.of weak limestones and siltstones. These rocks easily erode and this steep stream picks up a LOT of sediment. So much in fact that the channel gets choked with sediment and takes on the braided form you see here. The second is a meandering channel. In these channels there is a balance between the streams sediment load and it's capacity. Slightly less steep, this familiar channel form can be seen from the Mississippi to the Duwamish. The form is especially easy to see when the channel is cut through range land like this one. The rock is less friable (geology lingo for breakable) here, so less sediment loading. Now it's off to Vegas, but the geology that happens there may need to stay there.

13 April 2016

You Don't Need A Weatherman to Know Which Way the Wind Blows This rock formation is a good example of cross bedding. Frequently seen in eolian (wind derived) deposits, these are the lithified remains of ancient sand dunes. The coolest thing about them is that you can tell which way the wind was blowing when they were deposited. See the diagram. In the photo you can see how the wind patterns changed over the thousands of years it took to deposit these sands.
Zion National Park is only a 1.5 hour drive from Bryce, but is located in rock that is millions of years older (see cross section). Very similar to the stratigraphy at Arches, Zion boasts the uniform sandstone of the Navajo formation underlain by the less porous Kayenta formation. But here there is no network of vertical fissures (thus few fins), instead flowing water and wind are the erosional drivers. Deep canyons are carved through the Navajo by fast flowing streams laden with scouring sands and gravels. The hard rock Navajo holds vertical walls above the weaker Kayenta below. Much cleaner lines than seen at Bryce where weaker siltstones and limestones dominate.

12 April 2016

We made it to Bryce Canyon for a horse ride through these fantastic formations called hoodoos. These rocks of the Claron Formation consist of limestones, siltstones, mudstones, and dolomite, all of which weather at different rates. The mudstones is the weakest so it erodes under the more resistant layers creating the undulating formations. Of course, you also need the vertical cracking to create the fins. In this case the cracks were not formed by a rising salt dome, but instead by the uplift of the Laramide Orogeny.
The San Rafel Swell is another great example of an anticline. This arch of rock, once considered impenetrable, was only breached in 1970 when I-70 was constructed. In the diagram imagine we are on the right hand side looking into the anticline, that is our view in the photo. As we travel through the anticline we will see older and older rocks, and then the same layers in reverse on the way out. I think this is cool, my kids just want to go for a horseback ride.

11 April 2016

These amazing formation are carved out of the Entrada Sandstone which lies beneath the Dakota Sandstone (we saw that yesterday). How do they form, you may ask (let's assume you did). 1. You need a uniform porous sandstone underlain by a rock which is a bit less porous. 2. You need some vertical jointing. These form after the rock has solidified and it is pushed up from beneath. Can you say salt dome? 3. Just the right amount of rain. Too much and yucky vegetation would cover it all. Too little and you would not get enough erosive force. 4. A relatively earthquake free area. Because, you know, these things are pretty delicate. So, rain water runs down the vertical cracks and widens them. This creates fins of rocks. Within the fin, some of the water pools at the base on top of the less porous layer beneath. The pooled water dissolves the calcite which binds the sand grains of the Entrada. This causes more erosion at the base and the rock becomes undercut.

11 April 2016

The Great Salt Lake is only one example of a large terminal lake. Historically much of the West has been periodically covered by shallow inland seas. Like the Salt Lake, these seas have left behind huge salt deposits. With time, the layers of salt (hundreds of feet thick) become overlain by other sediments. Over eons different layers of sediment get deposited over the salt. So much so that the overlying pressure and heat found at depth act to solidify (aka lithify) the sediments into rock. But salt does not lithify, it melts, moves, recrystalized, melts again, and generally moves around to wherever the pressure is least. This inevitably leads to the salt being squeezed up toward the surface as a salt dome. The horizontal rock (in this case the Entrada Sandstone) above the dome gets pushed up and breaks in many vertical lines, like the top of a rising loaf of bread. These vertical cracks through horizontal sandstone provide ideal conditions for fins of sandstone to form, like these.
We passed out of Salt Lake City over the Wasatch Range and into Plateau Country. This is where things get really cool. Still drier, even more rocks are exposed here. In the distance is a buckled ridge (known as an anticline) of Dakota Sandstone framing the La Sal mountains (igneous rocks) in the distance. We are going to drive around the bend into the Colorado River basin and Arches National Park. Excited!
The Great Salt Lake is all that remains of the even greater Lake Bonneville. 20,000 years ago this lake occupied a third of Utah and was 1,100 feet deep. As the climate dried the lake level lowered until it became a terminal basin (a lake with no outlet). Under the desert heat the water poured in and evaporated, leaving behind dissolved minerals as level layers of salt. So level in fact that the Bonneville Speedway was built only 50 miles from here on the salt floor of historic Lake Bonneville. It is a great place to drive fast and straight! But more to the point. Salts deposits like these play a big role in the geology of what we will see ahead.

9 April 2016

Well, some weather does manage to make it past the mountains. Commonly in the form of thunderclouds like this one outside of Twin Falls, ID. The infrequent though intense rains which tend to occur in the desert southwest are the driving force behind much of the amazing erosional features we will see in Utah. Speaking of driving forces. The speed limit out here is 80!

9 April 2016

Here we go! From the heart of the Cascade Range at Snoqualmie Pass (https://goo.gl/maps/hERn6pXvjX82) we are headed to the arid eastside. This mountain range is HUGE and acts at a giant weather wall, moist on the west side and dry on the east. Orographic lifting is the process where moist air is forced into the cool upper atmosphere by mountains. The cool high altitude temperatures cause the moisture to condense and fall as rain. This dries the air out so by the time it moves over the ridge, it is dry. My sleeping kids in the car will take advantage of this effect during this dry eastside vacation.

6 April 2016

Here is the route for the trip. One Google map with distance (2557 miles!), and a geologic region map. Most of our destinations will be in the Colorado Plateau, a region of horizontal sedimentary rocks which has lifted thousands of feet as a single rigid block during the Laramide Orogeny. Bit of a mystery why it didn't get deformed into vertical peaks during the uplift. Maybe we will learn why when we get there. Leaving on Saturday!