Larrabee State Park - Rock Trail Earthcache EarthCache
Hidden : 6/21/2014
Difficulty:
2 out of 5
Terrain:
4 out of 5

Size: Size: large (large)

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Geocache Description:


Terrain Warning - there are 130 steep steps leading down to the cache from the Cyrus Gates Overlook parking area (thus the terrain rating of 4).

Getting There: Drive up Cleator Road to the top parking area at Cyrus Gates Overlook parking area.

Geologic Features of the Area: Cleator Road climbs up through the interior of a north-plunging syncline. The summit of Chuckanut Mountain, just above the trailhead, is the highpoint of the syncline's horseshoe; and the cliffs of the Rock Trail are right on the nose of a major syncline.

Along the Rock Trail, there are large fallen blocks of rock, cliffs with thin cross-bedded sandstone, cliffs with a conglomerate bed/sandstone combination, and the Tafoni Wall, as well as other interesting features.

The Hike: Begin your hike at the Cyrus Gates Overlook parking area, following the Rock Trail signage. The Rock Trail crosses a steep mountain bike trail that descends the ridge which marks the crest of the western limb of the syncline. Continue along the Rock Trail to the top of the steps where the trail sidehills southwest below the rim, parallel with the northwest-dipping beds that underlie Chuckanut Mountain.

One-quarter mile (400 m) from the trailhead is cross-bedded sandstone under an overhang (image below).



The Tafoni Wall (image below) is the last high cliff face along the Rock Trail and is reached one-half mile (800 m) from the trail head. The Tafoni Wall is the main focus of this EarthCache.



What is Tafoni? The base of the Rock Trail Tafoni Wall is pitted with various-sized holes weathered into the rock. They are not a remnant of older geologic conditions, but are continuing to form even now. These holes are called tafoni, a term derived either from the Corsican noun for ‘window’ or a verb, ‘to perforate’. Tafoni, natural pockets in rock, range in size from little pits to huge cavities and form a variety of shapes and patterns. The earliest known publication of the term tafoni was in 1882.

How Does Tafoni Form?
The sandstone in the Chuckanut Formation is permeable rock; the minerals that hold the sand grains together in the sandstone are water-soluble. A chemical weathering process called ‘solution’ dissolves the microscopic mineral cements that hold the sand grains together in the sandstone. Falling rain picks up carbon dioxide to make carbonic acid that dissolves the cements: H2O + CO2 —->> H2CO3. This is natural acid rain which has persisted for many, many years; long before air pollution, however at reduced levels.

Western Washington University Geology Department's George Mustoe determined that in the moist temperate Chuckanut environment, this weathering process involves algae that colonize rock surfaces. When algae form a micro-thin layer on rock surfaces, the algae inhibit absorption of water and so retard solution. Algae may or may not be readily apparent on rock surfaces; however, Chuckanut cliffs are pervasively colonized with algae -- the green film is visible on most rock surfaces unless they are freshly broken.

Acidic solution attacks an entire rock face, which may weather away evenly. When something occurs to damage the surface and remove, or kill the algae, solution attacks the cement immediately, and the sand grains fall off the steep surfaces. Each falling sand grain leaves a tiny pit in the rock which can be exploited by solution. The holes, only millimeters across, grow inward as solution migrates into the rock.

The surface damage could be caused by freeze-thaw expansion and fracture of the rock, the impact of a rockfall, a hard freeze that locally kills the algae, or a particularly permeable contact between rock layers. It may be that snow piled against the cliff bottom in cold winters plays a role in inhibiting the growth of algae. Notice that the more developed tafoni are at the base of the cliff.

Algae may re-establish on the sunny surfaces at the opening of the developing tafoni hole. Once algae recolonize the rock surface, solution is again retarded. However, algae may not grow well on shaded rock surfaces inside the entrance to a solution hole, so acidic water can dissolve cement more readily there and the holes migrate inward.

Since sandstone in the Chuckanut Formation is permeable rock, tafoni may grow laterally behind the algae-protected surface, and the developing tafone (singular of tafoni) is larger on the inside. The process is variable and complicated.



  1. This cache has an approved Permit to be placed at this location on property managed by the Washington State Parks and Recreation Commission. Visitors are responsible for acquainting themselves with policies and rules pertaining to State Park areas.
  2. The following items may not be placed in the cache: food; illegal substances; medications; personal hygiene products, pornographic materials; hazardous materials; or weapons of any type.
  3. By searching for the cache, visitors agree that they are responsible for their own actions, and acknowledge that neither the State of Washington nor the cache owner is responsible for any loss or injury that may occur in relation to such search.
  4. Report any incident, problem, or violation to State Parks staff.


QUALIFICATION REQUIREMENTS:

Submit an email with answers to the following questions. DO NOT POST THE ANSWERS IN YOUR LOG.

1. What discourages the formation of tafoni? (hint: read the cache page).

2. At the Tafoni Wall, select and measure the dimensions of at least one of the tafoni featured in the image on the cache page (maximum exterior width; maximum exterior height; maximum depth from inside the tafoni to the exterior lip). Provide measurements in inches or centimeters and reference tafoni 1 through 5 as follows: #1 upper left; #2 middle left; #3 lower left; #4 upper right; #5 lower right.

3. Peer inside a tafone. Describe what you see (i.e. color, texture, etc).

4. What is the name of the process that causes tafoni to form? (hint: read the cache page).

5. What evidence is there that new tafoni are forming on the Tafoni Wall?


REFERENCES:

Northwest Geology Field Trips, Dave Tucker, Research Associate, Western Washington University, Geology Department. Geology of the Rock Trail, Larrabee State Park click for link

Mustoe, G. E., 1982, The Origin of Honeycomb Weathering, Geology Society of America Bulletin, v. 93, p. 108-115.

http://en.wikipedia.org/wiki/Tafoni click for link

http://tafoni.com click for link


NOTE: For a longer hike, consider following the entire Rock Trail to where it intersects the South Lost Lake Trail -- a left turn takes you to the Lost Lake Trail; a right turn takes you on a long sweep to the Fragrance Lake area.

Additional Hints (Decrypt)

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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