Rock Quarry Shale Formation

Here at the Rock Quarry County Park Boat Ramp, there is a large shale/sandstone formation.  Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite.  The ratio of clay to other minerals is variable.  Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility. 

Shale typically exhibits varying degrees of fissility, breaking into thin layers, often splintery and usually parallel to the otherwise indistinguishable bedding plane because of parallel orientation of clay mineral flakes.  Non-fissile rocks of similar composition but made of particles smaller than 0.06 mm are described as mudstones (1/3 to 2/3 silt particles) or claystone (less than 1/3 silt).  Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are siltstones.  Shale is the most common sedimentary rock.

Shales are typically composed of variable amounts of clay minerals and quartz grains and the typical color is gray. Addition of variable amounts of minor constituents alters the color of the rock. Black shale results from the presence of greater than one percent carbonaceous material and indicates a reducing environment.[1] Black shale can also be referred to as black metal.[5] 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).[1] 

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 and purple.  Black shales contain organic material that sometimes breaks down to form natural gas or oil. Other shales can be crushed and mixed with water to produce clays that can be made into a variety of useful objects.

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.

Logging Requirements...

In order to log this cache, go to the posted coords and find the Rock Quarry Shale Formation.  Send me an E-Mail via my profile with the answers to the following questions. Do not post them to the log or your log will be deleted.

1.  Approximately how tall and wide is the shale formation visible here?

2.  How would you characterize the exposed rock surface?  Is it smooth or rough?  Durable or brittle?  What colors do you see in the rock face?

3.  What factors do you think led to the shale deposit being formed here at Rock Quarry?

4.  How thick are the little pieces of shale that have fallen off the face and are sitting on the ground?

5.  (Optional) Take a picture of yourself and your GPSr at the rock wall or some other interesting part of the Rock Quarry Boat Ramp or Park area and post it with your log.