Dinosaur Eggs?! Swiss Cheese? Omelet, Anyone?!
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While the trail switchbacks three times near the given coordinates, please wait until you the trail takes you to the base of the boulder here; you will NOT have to leave the trail to reach the GZ for this Earthcache. There is also a bench nearby for you to sit and contemplate your answers.
What is going on here?!
There are two types of weathering going on here!
Dinosaur Eggs
Although it may look like it, the rounded rocks here are not dinosaur eggs, nor are they cannonballs. These are actually concretions, which are masses of sedimentary rock. Concretions tend to be spherical or orb shaped, though sometimes they are irregular in shape. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'. Concretions are created in the sediment layers after the layer has been deposited, but before the rest of the sediment has hardened into rock. Then minerals brought to the area (i.e. via rainwater or erosion runoff) fill the space between porous sediment grains causing the soil to cement into sediment that is harder than the surrounding sedimentary strata. This causes the concretions to weather at a slower rate, so that over time the softer sedimentary rock erodes away more quickly, exposing these round, cannonball or dinosaur egg-like structures.
The boulder in front of you is made of sandstone. Sandstone isn’t uniform; better-cemented areas, called concretions, resist weathering, giving rise to decorations like the cannonballs. These cannonballs can also look like dinosaur eggs.
Swiss Cheese
A sculpting process called cavernous weathering—or tafoni weathering, from tafone (plural tafoni), the Corsican word for “cave”—has scooped out hollows at the boulder’s base. Note the intriguing honeycomb effect, called fretworks, stone lace, or stone lattice. Amid this horizontal array of pockets and concavities, a few unlikely bulges, called cannonballs, protrude from the rock. Limited by climate, rock type, and other circumstances, tafoni formations are uncommon. In this case, the stage was set some 30 million years ago when the Vaqueros Sandstone formed off Southern California’s shore. Deep sand sediments settled into a marine basin; then calcite, from the shells of marine animals, slowly cemented the sand bed together as it was carried north along the San Andreas Fault.
Tafoni weathering occurs in two phases, chemical and physical.
Chemical:
The chemical phase requires distinct wet and dry seasons, which our Mediterranean-like climate provides. Winter rains, slightly acidic from atmospheric carbon dioxide, soak into the rock. This mildly acidic water dissolves and begins to redistribute the calcite cement. Then, as summer’s warm, parched air slowly dries out the rock, the water, now laden with dissolved calcite, wicks outward. As water evaporates from the rock surface, the calcite it carried is left behind in the outer foot or so of the rock. Thus, summer drying concentrates the calcite cement, strengthening the outer rock and forming a hard outer crust, while weakening the inner rock.
Physical:
Anything that breaks that crust—falling branches, hail, windblown debris, passing animals—begins the physical erosion of the rock and exposes the weakened interior to accelerated weathering. Breaching that crust leads to the large caves. Then the same processes working on the weakened inner rock form the fretworks. The stone lace develops because calcite-laden water takes the path of least resistance, along cracks.
To Log This Earthcache:
1. Include GC7VDYD Dinosaur Eggs? Swiss Cheese? Omelet, Anyone?! and the names of any cachers that visited this earthcache with you.
2. Count the number of fully rounded concretions (do not include any broken ones). How many are there?
3. The following geological occurrences are all related to Tafoni weathering. Decide which ones are occurring here on the boulder to create the Tafoni weathering:
Salt Weathering:
a form of mechanical or physical weathering of rock. No chemical alteration of rock constituents is involved in salt weathering, but is caused by capillary rising ground water, eolian origin... a geological feature sculpted by the wind, sea water along rocky coasts, and/or atmospheric pollution.
Differential Cementation:
involves ions carried in groundwater chemically precipitating to form new crystalline material between sedimentary grains. The new pore-filling minerals form "bridges" between original sediment grains, thereby binding them together.
Structural Variation in Permeability:
weathering caused by wetting-drying, and/or freezing-thawing cycles.
Variation in Lithology (the study of the general physical appearance of a rocks):
the softer rocks are eroded or washed away more easily than the more resistant rock formations; case hardening and core softening Climatic Factors: presence of moisture, speed of wind etc.
4. Look at the large cavernous area. Just below it there is a section that looks like swiss cheese. This is Tafoni weathering. Estimate the width of the densest section (where the holes are small and close together). Your best guess is fine.
Nature needs thousands of years to sculpt these delicate decorations. Scrambling around the tafoni boulder endangers the formations, so please resist the temptation to climb on the boulder.
Resources:
https://en.wikipedia.org/wiki/Concretion
https://www.quora.com/How-are-tafoni-rock-formations-created
https://baynature.org/article/the-rock-in-the-redwood
https://wikivividly.com/wiki/Tafoni
Additional Hints
(Decrypt)
Ng gur onfr bs gur obhyqre naq ba gur genvy... fb AB ohfujnpxvat vf arprffnel!