Since a boy, I have trekked the hills surrounding the Napa Valley; especially the East side. What makes the Napa Valley floor productively rich and varied is its mixed sediments (alluvium) derived from both sides of the valley. The west side happens to have mostly marine sedimentary and metamorphic rock which differs widely from the lava flows and eruptive products of the east side. Speaking of the valleys variety of microclimates, it is brought about in part by the north-south orientation; The south end opens to the cool Bay air which contrasts to the hot and dry north end. And fog can blanket the valley from the west.
This earth cache is found on the East side of the valley where lava flows and beds of tuff (volcanic ash) make for awesome and unusual rock formations. The solitude and surprisingly wild setting offers the kind of spectacular scenic payoffs one would expect in more mountainous and wild terrain. It is well worth exploring. This trail is managed by the Napa County Regional Park and Open Space District.
E-Mail me your observations and answers. DO NOT POST YOUR ANSWERS or SPOILER PICTURES, and Please protect this fragile environment. The rock face can be defaced easily.
1. Give the GC code and Earthcache name.
2. Estimate the height of the rock formation at the posted coordinates.
3. What color is the well-bedded Volcanic ash (Tuff) at the base?
4. Is the content of the base a fine grain or coarse grain (See text below)? Give a simple reason for your answer. (Hint: grab a hand full of the matter at the bottom of a tafoni)
5. While standing back, give a rough estimate of the width and height of the largest tafoni you see.
Volcanic Ash and Tuff
Volcanic ash is a pyroclastic igneous rock, composed of fine grains less than 2mm in diameter. Pyroclastic means ‘fire-broken’, a name for rocks that have been blown apart by exploding gases. Ash exploded out of a volcano, may fall through the air and settle in beds, called ash-falls when unconsolidated, or tuffs when consolidated. The ash can vary in composition, for example it may be made up entirely of rock fragments. Color is white or grey powder when newly fallen. Ash-fall tuffs are composed of a mixture of rock fragments, crystals and glass chard.
Tafoni: What are they?
Tafoni are ellipsoidal, pan- to bowl-shaped, natural rock cavities. These cavernous weathering features include tiny pits, softball-sized cavities, truck-sized caves, and nested and cellular honeycomb forms. Tafoni typically develop on inclined or vertical surfaces and occur in groups. These exquisite and fascinating cavernous weathering landforms are present on the surfaces of many different kinds of rocks located in a multitude of geographic regions around the world.
Tafoni-like features comprise a class of relatively deep, rounded to elongated natural rock cavities bored primarily by rock weathering processes and secondarily by erosional processes. In the geological literature, the distinction between cavernous weathering features is unclear. I believe tafoni structures may include: small tafoni, pits, hollows, niches, recesses, alcoves, alveoles, alveolar weathering pits, gnammas, stone fretting, fretwork, recesses, honeycomb, and honeycomb weathering.
An ancient Tafoni formation located in the mountains near San Francisco, USA…..Author: Juha Huuskonen
To log this cache, you will need to hike the Oat Hill Mine Road trail; trailhead begins at the foothills on the North end of Calistoga. The trail difficulty is moderate to strenuous and will be around 8 miles round trip. It is accessible year round but please bear in mind it can be hot during the summer and cold during the winter; either way carry plenty of water. This is also rattlesnake country; watch carefully where you place your hands and feet.
An alternate and more remote access is out of Pope Valley by way of the Aetna Springs trailhead (GC1QNJV). The trailhead elevation begins at about 2160 feet which is an elevation advantage over the Oat Hill Mine Road trail. The last 4 miles of the Aetna Springs Road leading to the trailhead is not paved and some would recommend a high clearance vehicle for the last 1/2 mile before reaching adequate parking for 3 or 4 vehicles. It should be noted that hunting season begins in September and Aetna Springs trail to the Holmes place will be closed. Oat Hill Mine Road trail, the Palisades trail, and the Table Rock trail are always open.
http://napaoutdoors.org/parks-trails/oat-hill-mine-trail/ohmt-trail-brochure.pdf/view