NCAR Mesa Field Trip

Teachers in Geoscience

Field trip to the National Center for Atmospheric Research Mesa

Introduction to the Geology of Boulder, CO

The city of Boulder lies at a unique crossroads of geology in Colorado, to the east are the high plains and to the west are the Rocky Mountains. A little over a dozen miles to the west lies the continental divide, which acts as a separation for water that flows to the Gulf of California and the Pacific or to the Gulf of Mexico.

From anywhere in the city of Boulder or Boulder County one can view the most prominent geologic features: mountains. Rising sharply from the city's west side are the Flatirons (see Figure 1), a mountain range named for their likeness to the blacksmith's tool. These formations were uplifted during the Laramide orogeny that created the modern Rocky Mountains approximately 70 million years ago. However, the sedimentary rock that makes up the Flatirons is from the Fountain Formation and was laid down as fluvial deposits during the Pennsylvanian Period. The source of this sediment was the Ancestral Rockies. As they rose and eroded the sediment from the highland granite and gneiss came into the easterly basin and lithified into conglomerates, sandstones. During this time the climate was far warmer and moister than today's relatively dry climate. The evidence that led to this conclusion includes the relatively large texture of sediments including akrose sands and gravels as well as the paleogeographic evidence that indicates the continent was at a more equitorial location. Other notable exposures of the Fountain Formation include the Garden of the Gods near Colorado Springs and Red Rocks Park Amphitheater near Golden, CO.

Figure 1-The Flatirons

Source-University of Colorado.

Older rocks underlie the Flatirons and the city of Boulder. These include the igneous Boulder Creek Ganodiorite and the metamorphic Colorado metamorphic complex. These Precambrian rocks have been dated between 1.4 and 1.8 billion years old through the use of radioisotopic analysis. These old rocks formed at a time when Boulder would have been ocean front property!


Figure 2-Geologic Cross Section of Boulder

 
The Boulder geologic history book has some missing parts at this point in
time because the Cambrian, Ordovician, Silurian, Devonian, and Mississippian periods are only represented by an unconformity. As mentioned, the Pennsylvanian saw the formation of fluvial deposits that created the rock found in the Flatirons. This continued until the early Permian when the climate grew dry as Boulder became part of the western interior of Pangaea. This enabled large sand dunes to form with the source sediment again coming from the Ancestral Rockies. These dunes eventually lithified into the Lyon's Formation. Rocks cut from the Lyon's formation are easily viewed in some of Denver's red sidewalks and in CU-Boulder campus buildings. The Lykins formation also came to be during the Permian in the mudflat regions where some limestone, mudstone, and stramatolites can be found.

Figure 3-Cretaceous Interior Seaway

Source-Emporia State University

The Triassic left few pieces of evidence around Boulder despite ample evidence
of extinctions from elsewhere around the world. The Jurassic period included deposits of the Morrison Foundation-sandstone, siltstone, mudstone, and limestone. All were laid in freshwater environments including rivers and l akes. During the Cretaceous period the Dakota Group overlaid the Morrison Formation with a durable sandstone that today is visible as the Dakota Hogsback. Other rocks from the Dakota Group include shales and limestone that were deposited by the transgressions and regressions of the Cretaceous Interior Seaway.

The geologic story of Boulder comes becomes contemporary as the Tertiary and Quaternary periods saw the present day Rockies uplifted to form the Front Range to the west of Boulder. It was during this time period that the Indian Peaks rose to their prominence. This height provided the fodder for massive glaciers during the recent ice ages. The evidence today exists in the form of glacial till and remnant alpine glaciers such as the Arapahoe Glacier. This important geological feature still plays an important role in the lives of Boulder residents because it serves as a major source of drinking water.
 

Sources

  • Bridge, R. (2004). The Geology of Boulder County with 25 field trips. Boulder, CO: Lone Eagle Publications.
  • Mack, G.H., Suttner, L.J. (1977). Paleoclimate interpretation from a petrographic comparison of Holocene sands and the Fountain Formation (Pennsylvanian) in the Colorado Front Range. Journal of Sedimentary Research, 47.
  • Murphy, S. (2007). Geology of the Boulder Area. Retrieved from http://bcn.boulder.co.us/basin/watershed/geology/index.html.
     

Introduction to Field Trip Activiy

The National Center for Atmospheric Research lab and visitors center sits atop the mesa west minutes west of the city of Boulder. The visitor's center is a great location to begin or end a field trip in order to learn a little atmospheric science before stepping outside for a tour of the local geological features. As an added bonus all the exhibits are free.
 
To reach the mesa travel to the intersection of Broadway and Table Mesa. Travel west on Table Mesa towards the foothills. See the map below for detailed directions.
 
 

National Center for Atmospheric Research Mesa Field Trip

 

The NCAR Mesa sits at 6200 feet above sea level atop recently deposited gravel. Below the gravel is the deep layer of Pierre Shale. Surrounding the mesa you can find faults, many varieties of rock, and several different formations all within easy walking distance.
 
In this activity students will hike approximately 2.5 miles, investigate rock formations, enjoy beautiful views, and complete the assignment below for approximately 3 hours.

Sources

Activity

Stop #1
 
  • Walk west towards the mountains from the parking lot and find a large rock marked with the words "Walter Roberts Nature Trail". This rock shows features of layered cross-bedding. Cross bedding occurs as wind or water currents deposit sediment. Sand is generally the largest sediment that can be cross bedded by wind currents and gravel is the largest sediment that can be deposited by river currents. The direction of the cross bedding can tell you the direction of the current as the beds angle downward in the direction of the flow. See figure 4 below. Rocks gain unique coloring from the minerals that make them up. Potassium feldspars tend to be pinkish, quartz tends to be whitish, and hematite tends to be red.

Figure 4-Cross Bedding

Source-University of Wisconsin

a. Record your observations of the rock.
b. What type of rock do you think this is? How do you know?
c. Examine grain size of this rock. Was this sediment deposited by wind or water?
d. What direction was the flow that deposited this sediment?
e. What minerals do you observe in this rock?
 
Stop #2
 
  • Continue walking up the trail to the fork with a trail marker that says "Mesa Trail". Take the trail to the left and stop at the sign that reads "Fire and Drought". Observe the view beyond the "Fire and Drought" sign and try to spot Bear Peak, it is the most prominant peak in this view of the Flatirons (see figure 5). The large, brown, flat, angled, blocks are the Flatirons from the Fountain Formation. The pine covered ridge across the valley below Bear Peak is the Dakota Hogsback.

Figure 5-View of Bear Peak

 
a. What process created the valley feature directly in front of you? What evidence do you have to support this claim?
b. The Dakota Hogsback directly past the valley and the Flatirons behind the Hogsback are very prominant compared to the other rock features in your field of view. What is do you think is different about the Dakota Hogsback and the Flatirons from the other rocks that makes them so prominent?
c. Turn towards your left and find downtown Denver in the distance and Boulder in the foreground. The rock formation between you and Boulder is called Pierre Shale. It is also visible beyond between Boulder and Denver. What can you say about the low lying Pierre Shale's resistance to weathering and erosion compared
to the Flatirons?
 
Stop #3
 
  • Continue walking up the trail until you reach another trail map. At the trail map take the left trail and follow it until you reach the sign that reads "Mountain Waves". Look to the northwest towards the flatirons and a quarry just above the trees in the forefront.

Figure 6-Lyons Sandstone Quarry

 
a. From this viewpoint you can see the Lyons Sandstone quarry. This is where people extracted blocks of the pretty red sandstone for use in building structures before the area became a park. The sandstone is located on top of the flatirons. According to the law of superposition, which rock is older, the sandstone or the Flat irons?
b. This viewpoint also clearly shows the angled flatirons. What could have caused the Flatirons to rise at such an angle?
c. There are two distinctive rows of Flatirons in view. These rows were created by the Maxwell Fault. Explain how the fault could have created two rows of Flatirons.
d. What direction did the rocks on the right (east side) of the Maxwell Fault move relative to the rocks on the rock on the left (west side) of the Maxwell Fault?
e. What direction did the rocks on the left (west side) of the Boulder fault move relative to the rocks on the right (east side) of the Boulder fault?
f. The gravel is the youngest rock in figure 7 and then Pierre Shale is the next youngest. As you walk west (left) throughout this field trip will you be going back in time or forward in time based upon the age of rocks underneath you?
 

Figure 7-Faults of Boulder

 
Stop #4
 
  • Walk back to the Mesa trail, travel past the first swithback and observe the rocks surrounding the second switchback. Continue to the juniper grove and make more observations of the surrounding rock. See if you can spot the grey colored limestone, white colored chalks, and black colored shales.

Figure 8-Shells of Boulder

 
 
a. Record your observations of the limestone, chalk, and shale.
b. Limestone, chalk, and shale are only deposited in water. What does this tell you about how this area of Colorado was different 60-80 million years ago when this rock was deposited?
c. Pick up a few pieces of limestone and see if you can find any fossils. Based on figure 8, how old do you think the fossils are that you found?
 
Stop #5
 
  • Continue down the path into the valley until you reach a wooden fence. Look around to try and find the following rocks: dark grey shale (a type of mudstone) and yellow shale (another type of mudstone called bentonite). The bentonite is actually a sedimentary rock made up of clay and ash.

Figure 9-Bentonite 

  
 
a. Record your observations of the dark grey shale and the bentonite (yellow shale).
b. Both rocks are made up of small sediments called clay. Clay is only deposited in very slow moving water. How was this location different when the shales were deposited?
c. Pick up some of the bentonite in your hand. Pour some water from your water bottle onto the bentonite and record your observations.
d. Bentonites unique properties that you just observed make it useful for all sorts of things, from making homebrew, to making concrete, to making liners for wastewater ponds. Would you want to build your home on bentonite? Explain.
 
Stop #6
  • Continue walking down the trail to the bottom of the valley and up switchbacks to the top of the hogsback on the ridge. All around the water tank here you will find sandstone rocks with more examples of cross bedding. This rock was deposited in a different environment that the boulder at stop one and thus the cross bedding looks much different. 
a. Find the some cross bedding with wavy lines. These are called wave ripple marks. And they preserve the pattern of waves that flowed over the sand when it was sediment. What type of environment do you think this rock was deposited in?
b.  Find the cross beds with squiggles in it. These are created when something disturbs the layers of sand below the water surface. What do you think could have caused these? (Hint-they are a type of fossil.)
 
Stop #7
  • Continue to walk west on the trail to the next ridge. Walk down the trail from the ridge to a fork. Take the left fork. Continue on this trail until it intersects with the Mesa Trail and the Mallory Cave Trail. Take the Mallory Cave Trail. Look to the west for a large section of white rocks on a small ridge. A chunk of this rock has fallen from the ridge and is near the trail. Find that grayish-white chunk. It is easy to spot thanks to the brightly colored lichens.

Figure 10-Stromatolites of Shark Bay Australia

 

Source: The Internet Encyclopedia of Science

a. Sketch a side view of the grayish white block of rock.
b. This block is made up of fossilized stromatolites. Stromatolites are mats of cyanobacteria that live in shallow salt water and photosynthesize. What do you think this environment looked like when the stromatolites were living?
c. How do you think stromatolites and other photosynthesizing organisms changed the constitution of the atmosphere?
 
Stop 8
 
  • Continue to explore up the trail (it continues up to Mallory Cave for about a mile) if time permits or return to the NCAR visitors' center via the trail you came up. Upon returning to the visitors' center choose one of the following exhibits to attend and complete a KWL chart (what you KNOW, what you WANT to know, and what you LEARNED) based upon the topic of your choosing from a-d below.

Figure 11-NCAR Visitors' Center

 

Source-University of Colorado

 
a. The Weather Gallery Hands-On exhibits can help you learn more about lightning, hurricanes, and other exciting weather phenomena.
b. You can view the Climate Discovery Center to learn about Earth's past, present, and future climates.
c. For a limited time you can view the traveling exhibit Greenhouse Gas Photos to see climate change's impacts on the world.
d. You can also stay outside and learn more about local weather on the Walter Orr Roberts Weather Trail.
 

Sources:

 
     
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