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Mirror Lake, Mirror Meadow - Which one is it?

A cache by TerryDad2 Send Message to Owner Message this owner
Hidden : 4/27/2009
Difficulty:
3 out of 5
Terrain:
1.5 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

A rockfall blocked the Tenaya Creek to create Mirror Lake. Over time, the lake has filled in and it has become Mirror Meadow for much of the year.

The closest shuttle stop is No. 17. In the winter months, the shuttles do not make it up that far. There is a paved road up to the start of the lake/meadow and then becomes hard packed dirt. Vehicles with handicapped placards can drive up the steep hill to within a few hundred feet of the coordinates.

Mirror Lake formed after a rockfall from Washington Column (the north side of the creek behind the restrooms) blocked the flow of Tenaya Creek. Tree size, lichen growth, and evidence of Native American occupation of caves that were formed in the rockfall suggest that the evant occurred about 300 to 500 years ago.

Initially the lake extended about 1.2 miles (2 km) upstream but from the moment the lake formed, it began filling in. Sediment that is eroded from the fast flowing water of Tenaya creek up stream are deposited in the lake as the flow of water slows decreasing the capacity of the water to move large particles.

The upstream end of the lake filled was the first area to fill in as a delta extended into the lake from where Tenaya Creek entered Mirror Lake. This process of lake infilling is the natural evolution of every lake, natural or man-made.

The lake would likely have already filled in if the National Park Service had not attempted to maintain the lake. Small dams were constructed upstream to catch the sediment before it reached the lake. They also dredged the lake using the sand on the roads during the winter. However, since about 1970, these practices were abandoned allowing natural processes to begin filling the lake again.

Now Mirror Lake forms only during high water and is mostly a meadow at other times of the year.

On March 28, 2009 another rock fall occured on the Half Dome side of the canyon. This rockfall is the largest event in Yosemite National Park since the 1987 . Even larger than the rockfall near Happy Isles. Take note of that rockfall as some of the logging questions are about it.

Logging requirements (note that the answers to these questions are not on a sign. They are your own observations):
Send me a note with :

  1. The text "GC1QN2X Mirror Lake, Mirror Meadow - Which one is it?" on the first line
  2. The number of people in your group.
  3. What does the size of the particles that are now filling lake bottom say about the speed of the water that deposited them?
  4. How does the shape of the rocks that make up the dam show that they came from a rockfall and not a glacier?
  5. Since the upstream end of the lake filled in first, the trees at that end should be older. Can you see a difference in the age of the trees between the area near the rockfall, the most recent rockfall and further updtream?
  6. Did the recent rock fall from Ahwiyah Point form another dam in the river creating a new lake?

The above information was compiled from the following sources:

  • Wieczorek , Gerald F. 2002, Catastrophic rockfalls and rockslides in the Sierra Nevada, USA Geological Society of America, Reviews in Engineering Geology, Volume XV, 2002
  • http://virtual.yosemite.cc.ca.us/ghayes/roadside.htm http://www.yosemite.ca.us/faq/4.html
  • http://www.nps.gov/yose/parknews/ahwiyahrockfall.htm

Trial listing approved by
Yosemite National Park


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Additional Hints (Decrypt)

Ab arrq gb yrnir gur genvy gb nafjre nal bs gur dhrfgvbaf

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)



 

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