Your job is to write a short paper that describes the geological history of the Grand Canyon area based on the information given below, what you see in some pictures, and what you have read in your book. You do this in two steps. First you look at the pictures and answer some questions. Then you put your thoughts together and write a 3-5 page paper that describes the history of the Grand Canyon.
You can write a "science" paper, or your history can read like a story (e.g., "Once upon a time, about 2 billion years ago....then a great sea came in and covered the land...rabbits and deer inhabited the area and all were happy...but, alas, they were flooded by another sea that encroached on the land...").
Be sure to use real ages (millions of years) in your story. Don't just say "in the Mesozoic" or "in the Permian" - be specific about how many years ago.
Be sure to consider all evidence and to incorporate the uncomformities in you story.
Warning: I have "modified" some of the information, so if you go to a webpage where someone else has described the history of the Grand Canyon, you will find the WRONG answer!
Fabulous prizes for the best story!
Part A. Pictures
First, go to the slide show by clicking here. Then, answer the following questions. A good source of information is the stratigraphic column (at the bottom of this web page) that describes the rock formations. Turn in your answers for these questions separate from your geological history:
Picture #1. This photos shows some cliffs
and some eroded debris covered slopes.
1. What formations make up the cliffs and what make up the slopes?
2. What kinds of rocks are the formations?
3. Which kinds of rocks erode fastest?
4. For each of the rock types: in what sorts of environments were they deposited?
Picture #2 and Picture #3. Note that the
Supai Group is highly variable. Different layers have different
kinds of rocks, so the slope has outcrop and scree in alternating
5. What does this tell you about the environment when the Supai group sediments were deposited?
Picture #4 and Picture #5. The Redwall limestone
is thought by many people to have formed in a clear open sea with
moderate depths of water. Mostly it is made of calcite. At its
base it grades into the Temple Butte limestone, which is made
of dolomite. The Redwall limestone contains fossils of dead sea
critters, including brachiopods, corals, crinoids and bryozoans.
6. What is dolomite and how does it differ from calcite?
7. What was the environment where the Redwall limestone was deposited?
Picture #6 and Picture #7. Here you see
the Redwall and other formations forming cliffs. Some distance
below is the Tonto Platform. The rocks of the Tonto Platform contain
rare fossils of shellfish that lived in muddy tidal flats.
8. How steep is the slope of the Tonto Platform? Why?
9. What formation forms the platform, and what kind of rock is it?
Picture #8 shows the Tapeats Sandstone.
Ripple marks, crustacean tracks and worm trails have been found
in this sandstone. Picture #9 is another view of the Tapeats,
with Precambrian rocks below.
10. Describe the layers in the Tapeats Sandstone; compare with the layering in the other rocks you have seen in previous pictures.
11.Why are there so many layers?
12. What are crustaceans and what are crustatcean tracks? What does their presence tell you about the environmnet where the Tapeats sandstone was deposited?
13. Tough question: why is bedding (layers) apparent in the Tapeats Sandstone (and rocks above) but not in the Precambrian rocks below?
Picture 12-14. These pictures show the lowest
parts of the Canyon and the Colorado River.
14. What kinds of rock are in the lowest parts of the Canyon?
15. Why do these rocks near the Colorado River NOT show bedding like the ones above?
16. What color is the Colorado river? Why is it so cloudy? Speculate.
Part B. Figures
To give you more guidance, check out the figures and captions below - from Breed et al. (Geologic Guide to the Bright Angel Trail) and from Harris et al. (Geology of National Parks)
<==Drawing at left is an idealized cross section of the south wall of the Grand Canyon (Breed et al.)
Part C. Geologic Column
Below is a geologic column for the Grand Canyon. I have given descriptions of the various formations. This is the information that geologists use to construct geological histories.
Note that squiggly lines in the column shown above and dashed lines below indicate uncomformities. As you know, that means that something is missing, possibly because the land was high and being eroded at that time! I have only marked the most significant uncomformities. They occur above and below the Zoroaster Granite, at the base of the Temple Butte Limestone, and at the base of the Surprise Canyon Formation.
|Permian||Kaibab Limestone||Gray, sandy, massive, limestone up to 320' thick. Abundant fossils include corals, squids, sponges, and shellfish.|
|Toroweap Formation||Red and Yellow sandstones at the top and bottom of the formation, and some limestone between. Common fossils include corals, sponges, sharks teeth, and many kinds of clams, etc.|
|Coconino Sandstone||A massive white to buff colored, crossbedded sandstone about 400' thick. Almost all quartz, well sorted, fine grained, and displays huge aeolian crossbedding. Trails of quadrupeds, either reptiles or amphibians, have been found.|
|Hermit Shale||100-300' thick, predominantly shale but also includes some sandstone strata. The sandstones have a deep red color. Some shale shows mudcracks and ripple marks. Fossils of plants, mostly ferns, and quadruped footprints have been found.|
|1000' thick series (group) of alternating red crossbedded sandstones and shales. The upper part of the group is non-marine and tracks of quadrupeds are found on bed tops. These tracks are believed to have been made by amphibians or primitive reptiles. Sediments appear in many places to be thin beds spread over wide areas in short periods of time. The lower part of the Supai includes calcareous sandstones and shales which many believe are of marine origin.|
Surprise Canyon Formation
---- unconformity ----
|Variable deposits include sands and conglomerates. Cross bedding is common but localized. Great horizontal variation in stratigraphy.|
|Redwall Limestone||Thick to massively bedded, bluish-gray limestone beds up to 600' high. The most conspicuous cliff in the canyon. It appears red, but that is only on the surface. Various invertebrates, including corals, shellfish, and crinoids are present as fossils.|
Temple Butte Limestone
---- unconformity ----
|A calcareous sandstone, lavender to purplish colored, 50-100' thick. Fossils of armored fish, corals, shellfish and snails have been found.|
The Muav Fm. is a gray to buff limestone 300-400' thick. At its
base, the limestone is interbedded with green shale and sandstone.
At its top the limestone grades into brown shales and sandstone.
The Bright Angel Shale is mostly thinly bedded sandstones, but conspicuous micaceous shales and dolomite beds are also present. It varies between 350 and 400' thick.
The Tapeats Sandstone is massive, coarse to medium grained, 100-300' thick. It is generally chocolate brown, but is lighter colored in some areas. Cross beds are very common. Ripple marks, showing strong currents in one direction can also be found. Trilobite (an extinct early crab like critter) trails and worm tracks are present.
|Late and Middle Precambrian||Dox Sandstone||Sandstone mixed with limey shale. Ripple marks and cross bedding are present. Up to 1700' thick but variable.|
|Shinumo Quartzite||Thick bedded, massive, white, variable color, 1100' thick in some places. Many cross beds and ripple marks on a fine scale. May form cliffs locally.|
|Hakatai Shale||800' thick reddish and vermilion mudstones and shales interbedded with minor sandstone. Ripple marks, mud cracks, raindrop impressions are common. This formation is generally eroded to a smooth slope.|
---- unconformity ----
|Mostly gray dolostones, weathered dark brown in places. Up to 200' thick. Interbedded shales and sandstones are present, often showing ripple marks. Fossil algae have been found.|
---- unconformity ----
|Although stratigraphically below the Vishnu Schist in many parts of the canyon, this intrusive igneous rocks is actually younger in age. Granites of this sort are associated with uplift and formation of mountain ranges.|
|Vishnu Schist||Oldest rock in the canyon. Formed by metamorphism of rocks that were originally sedimentary. The metamorphism occurred after the rocks were buried to great depth by mountain building. This formation is now tilted up and in places approaches vertical. Thickness unknown.|