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Kincaid Park's Eolian Landscape - Cliff Head Dune

A cache by NorthWes Send Message to Owner Message this owner
Hidden : 3/20/2007
3 out of 5
3.5 out of 5

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Geocache Description:

Eolian processes are the eroding/transporting/depositing functions used by the winds to shape the surface of the Earth and other planets. This site demonstrates the impact of erosion, transportation and deposition of soils caused by the wind.

This is an EarthCache - without container or logbook - where you'll observe, document through measurements and photographs, 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 in this section of Kincaid Park. Read all the requirements for completing this cache before you visit the park, and equip yourself with the necessary tools (including a camera).

This aerial photograph shows Kincaid's Motocross Park, located off Jodhpur Rd beyond the west end of Dimond Blvd. During its earliest years of use by homesteaders and construction companies this pit proved to be an excellent source of relatively clean sand and was mined extensively. Today the pit's vegetation-free surface is the only place inside the city where riders can use their snowmobiles, four-wheelers and dirt bikes, with organized racing held in the summer months. The extensive motorized activity in the pit coupled with the lack of vegetation has contributed directly to the creation of a moving coastal Cliff Head Dune.

Your parking area is at right, on the east side of the complex. From your vehicle you can see the large dune rising to the west beyond the motocross track. Walk downhill on the gated road leading south down towards the bluff and the complex below. Move westwards towards the dune itself, looking for the chainlink fence bordering the actual track area's south side. Are there clues to erosive wind activity at the base of this fence? Measure the height from the top of the fencepost's concrete anchor to the ground below at its largest separation (thanks to the wind, winter snows probably won't hamper your view). This distance shows that soil mass has been eroded from the loosened sand through a process called deflation, where loose fine-grained particles are removed by the wind. Scouring of surfaces by the wind may produce areas known as blowouts, where soils are disturbed or less-compacted and are thus more susceptible to wind transport. Blowouts are common in areas where human activity has occurred across exposed non-bedrock soil surfaces without any subsequent stabilization of soils.

Continue westward up and across the dune's slope, walking towards the treeline to the left of the head of the dune. If you've arrived on a windy day you'll witness first-hand the wind transporting particles up the dune through suspension, saltation and creep. Small particles under 0.2 millimeters in diameter are held aloft in a process known as suspension, where they can be borne aloft indefinitely. Winds blowing across the braided areas, mud flats and sand bars on glacier-fed rivers can pick up enough material to create dust storms - a familiar occurrence on the Knik River near Palmer AK. Saltation lifts sand-sized particles no more a centimeter above the ground in a series of short hops or skips downwind. During saltation the particles bump and push larger grains along the surface in a process known as creep, which can account for as much as 25% of grain movement across exposed surfaces.

Deposition - the accumulation of wind-borne sediments into mounds or ridges - provides clues about wind direction, consistency and speed. Dunes are built up through the process of deposition as materials move up the gentle upwind slope. When particles at the crest of the dune exceed the angle of repose (steepest angle at which materials will remain cohesive), they spill over in a tiny landslide or avalanche that reforms the slipface - the steep upwind side of the dune lying perpendicular to the wind's direction. As the avalanching continues, the dune moves in the direction of the wind. Coastal cliff head dunes (such as this one) are formed where materials loosened by wind and wave action are lifted onto the face of the shoreline bluff and deposited in growing mounds of material. The cliff head dune may be very mobile or stabilized by obstacles such as vegetation or sand fences. When winds blow consistently from one direction the dune will be simple in shape with slipfaces uniformly perpendicular to the wind.

As you climb the dune, move west off the edge and view the head (crest) of the dune from its southwest side at the treeline. Looking to the east-southeast you'll see across the Anchorage Coastal Refuge and right down the mouth of Turnagain Arm. At low tide extensive silt flats (more than 900 feet deep in places to bedrock below) are exposed to the Eolian processes of the wind. As the glacier-fed rivers at the head of Turnagain Arm continue to deposit silt onto the flats, the wind continues its work of reshaping those flats at low tide. Much of the upper soils of Kincaid Park consist of fine particles lifted from the flats and deposited on the glacial till gravel outwash sub-base of the bluff.

To demonstrate your acquisition of knowledge about the Eolian forces at work in this section of Kincaid Park, please answer these questions briefly in an email to me through the website. Post a photo in your log of yourself at the dune (not necessarily the top of the dune) with the crest visible in the photo and/or at the chain link fence deflation site, but don't post your answers in your log. Photo is not required - but it helps record your passage across this landscape! Report in the online log how many were in your party during your visit. Logs not conforming to these requirements will be deleted.

1) Based on your observations, what is the depth of soil that has apparently deflated from under the chain link fence?
2) Observe the exposed dune's crest and report:
A) its direction of travel (using directions such as east or SE)
B) your estimate of its height above the parking area
C) the prevailing direction of the wind at this location

3) What bio-community is the dune impacting with its travel, and how?
4) Why is this particular portion of the southern cliff head dune system here at Kincaid Park moving, while the rest of the cliff head dunes to the west are relatively stable?

The above information was compiled from the following sources:
A.S. Walker, "Deserts: Geology and Resources" (online edition), US Geological Survey, 1998 USGS Geologic Information General Interest publications,
L.M. Dilley & T. Dilley, "Guidebook to Geology of Anchorage, Alaska", 2000 (1st edition) Anchorage, AK

Cache placed by permission of the Municipality of Anchorage's Kincaid Park.

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