Shale along the Trail EarthCache
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This Earthcache is along the trail from Pondorosa Bridge to Codfish Falls. The trail is easy with very little elevation
change. We hiked it with my 3 year old son. He made it all the way without complaint.
At the posted coords you will find a deposit of Shale along the Trail.
Texture
Shale is a fine-grained, sedimentary rock made up of flakes of clay minerals and tiny fragments of other minerals,
especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by thin laminae or
parallel layering or bedding. Shale is the most common sedimentary rock.
Composition and color
Shales are usually made up of different amounts of clay minerals and quartz grains and the typical color is gray.
Addition of different amounts of minor constituents changes the color of the rock. Black shale results from the
presence of greater than one percent carbonaceous material and indicates a reducing environment. Red, brown and
green colors are indicative of ferric oxide (hematite - reds), iron hydroxide (goethite - browns and limonite - yellow),
or micaceous minerals (chlorite, biotite and illite - greens).
Clays are the major constituent of shales and other mudrocks. The clay minerals represented are largely kaolinite,
montmorillonite and illite. Clay minerals of Late Tertiary mudstones are expandable smectites whereas in older rocks
especially in mid to early Paleozoic shales illites predominate. The transformation of smectite to illite produces silica,
sodium, calcium, magnesium, iron and water. These released elements form authigenic quartz, chert, calcite,
dolomite, ankerite, hematite and albite, all trace to minor (except quartz) minerals found in shales and other
mudrocks. Shales and mudrocks contain roughly 95 percent of the organic matter in all sedimentary rocks. However, this
amounts to less than one percent by mass in an average shale. Black shales which form in anoxic conditions contain
reduced free carbon along with ferrous iron (Fe2+) and sulfur (S2-). Pyrite and amorphous iron sulfide along with
carbon produce the black coloration.
Formation
The process in the rock cycle which forms shale is compaction. The fine particles that compose shale can remain
suspended in water long after the larger and denser particles of sand have deposited. Shales are typically deposited
in very slow moving water and are often found in lakes and lagoonal deposits, in river deltas, on floodplains
and offshore from beach sands. They can also be deposited on the continental shelf, in relatively deep, quiet water.
'Black shales' are dark, as a result of being especially rich in unoxidized carbon. Common in some Paleozoic and
Mesozoic strata, black shales were deposited in anoxic, reducing environments, such as in stagnant water columns.
Some black shales contain abundant heavy metals such as molybdenum, uranium, vanadium, and zinc. The enriched
values are of controversial origin, having been alternatively attributed to input from hydrothermal fluids during or after
sedimentation or to slow accumulation from sea water over long periods of sedimentation.
Fossils, animal tracks/burrows and even raindrop impact craters are sometimes preserved on shale bedding surfaces.
Shales may also contain concretions.
Shales that are subject to heat and pressure of metamorphism alter into a hard, fissile, metamorphic rock known as
slate. With continued increase in metamorphic grade the sequence is phyllite, then schist and finally to gneiss.
Logging Requirements...
In order to log this cache, go to the posted coords and find the Shale along the Trail.
Send me an E-Mail via my profile with the answers to the following questions.
1. Approximately how tall and how wide is the Shale face.
2. How thick are the little pieces of shale that have fallen off the face and are sitting on the ground.
3. (OPTIONAL) Post a picture of you with your GPSr and the river in the background. Please don't post pictures of the shale face.
Additional Hints
(No hints available.)