Continuing our series of Earthcaches in the Anchorage area, the
coordinates take you to the Williwaw Campground East of Portage. As
you travel you will see glaciers almost all around you. There are
several on the right as you travel to Portage; they are Scout,
Explorer, and Byron Glaciers. There are also areas where glaciers
once existed. After going through the first tunnel to Whittier,
look straight ahead and up to see part of the Harding Ice Cap. To
the left is Burns Glacier which once merged with the Portage
glacier at the Begich-Boggs Visitor Center.
Glaciers are responsible for the majority of the land features
around us in this part of Alaska. Turnagain Arm and Cook Inlet are
both the products of glacial erosion. The visitor center at Portage
sits on a terminal glacial moraine. Driving toward Byron Glacier
and just before a turnout on the left is a moraine of large rock on
the right. It is possibly a terminal moraine of the Byron Glacier.
In Anchorage, the topography of Russian Jack Park was created by
glaciers.
Morraines will be the subject of a future earthcache. This
earthcache is about Glacial Erratics. A glacial erratic is a
piece of rock that deviates from the size and type of rock native
to the area in which it rests; the name "erratic” is based on the
errant location of these boulders. These rocks were carried to
their current locations by glacial ice often over hundreds of
miles.
"The Big Rock" is the world's largest known glacial erratic--rock
transported far from its place of origin by glacial ice. Big Rock,
also known as the Okotoks Erratic, is the largest rock in the
Foothills Erratics Train, a group of rocks that were carried by ice
along the mountain front and dropped as the glacier melted some
10,000 years ago. The erratics lie in a narrow band extending from
Jasper National Park to northern Montana. The Okotoks Erratic
weighs 16,500 tons. It measures 9 metres high, 41 metres long and
18 metres wide. The rock has been eroded into pieces, but is still
a large landmark on the flat prairie.
There are probably hundreds of erratics in the Anchorage Bowl. Two
of the most familiar are in the vicinity of the Tarzania Cache
(CG9587).The size of these boulders indicate the power of a
glacier. Imagine the weight of these erratics and that in Williwaw
Campground and envision them being carried along by the
glacier.
The erratic in the Williwaw Campground has markings that likely
point to its origin.
This is an EarthCache - without
container or logbook - where you'll observe, document through
measurements and photographs or other means and report your
findings, via email, to earn credit for this cache. Logs which do
not conform to these requirements will be deleted, as this cache
requires a demonstrated acquisition of knowledge about the geologic
forces at work. Read all the requirements for completing
this cache before you visit the area, and equip yourself with the
necessary tools (including a camera).
To demonstrate your acquisition of
knowledge of this erratic, please answer these questions briefly in
an email to me through the geocaching.com website. Post a photo in
your log of yourself at the erratic, but don't post your answers in
your log. Report in the online log how many were in your party
during your visit. Logs not
conforming to these requirements will be deleted.
1. Measure the height, width and length of the erratic. The rock is
probably gneiss. Using this Tool, determine the weight of this erratic.
2. What marks on the erratic might tell its origin?
3. Could the marks mean something else?
4. Where do you believe the erratic originated?
Thanks for participating in this earthcache. For more
information see:
L.M. Dilley & T. Dilley,
"Guidebook to Geology of Anchorage, Alaska", 2000 (1st edition)
Anchorage, AK
Cathy Connor; Daniel O’Haire, Roadside
Geology of Alaska, 1988 Missoula, MT
While you are here, this benchmark is nearby.
The Geological Society of America (GSA) EarthCache Project -
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Master Program: earn Bronze / Silver / Gold / Platinum status,
with an EarthCache Master Pin awarded at each level!