Located at this spot are a couple of rocks that don't look like your typical rocks. In fact, they kind of look like something meteoritic, but I highly doubt that's where these came from. So I did a little research to arrive at my own hypothesis of how these rocks were formed, here’s a bit of what I found....
Key terms used to talk about sedimentary rocks:
Classification:
Clastic sedimentary rocks are rocks composed of solid fragments of preexisting rock. Clast: a single such fragment. Fragments may be rock fragments including several minerals or individual mineral grains.
Chemical sedimentary rocks are rocks composed of material that was transported in solution and directly precipitated from solution. These precipitates may have the texture of a crystalline crust or may form around small particles that have a clastic texture, i.e. they resemble clastic rocks.
- Evaporites: When we confine a body of sea water, the water evaporates, leaving increasingly concentrated brine until the salt precipitates leaving lake beds marked by crusts of minerals.
- Precipitated limestone: Rocks composed of calcite and aragonite are called limestone, the majority of which is biochemical, but in some cases, it forms as a direct precipitate, such as in travertine which precipitates and are deposited near hot springs.
- "Pseudoclastic" chemical sedimentary rocks:
- Ooids - (="little eggs") Sand-sized granules of calcium carbonate that form in shallow tropical waters. Ooids form around a nucleus of a mineral grain or shell fragment. Concentric layers composed of calcium carbonate precipitate out and progressively coat the nucleus, forming a pattern almost like tree rings.
- Pisolites: rock made of pisoids, which are concretionary grains – often of calcium carbonate, but sometimes of rarer minerals – which resemble ooids but are always more than 2 mm in diameter. These grains are approximately spherical and have concentric layers reaching 10 mm in diameter. The name derives from the Hellenic word for pea.
Biochemical (or "biogenic") sedimentary rocks are the remains of organisms. One common form is limestone which is formed from the remains of marine organisms made of calcium carbonate, the main component of eggshells, gastropod shells, shellfish skeletons and pearls. Another is coal which is carbon from compressed remains of land plants.
Size:
The Udden-Wentworth grain-size scale describes the sizes of the grains in these materials:
- Clay Finer than .0039 mm
- Silt .0039 - .062 mm
- Sand 2 - .062 mm
- Pebble 2 – 64 mm
- Cobble 64 – 256 mm
- Boulder Courser than 256 mm
Conglomerate & Breccia: Sedimentary rocks made primarily of pebble sized (2 mm.) clasts or larger. Rocks are known as conglomerate if the clasts are well rounded, and breccia if they are angular. Conglomerates form in high-energy environments such as fast-flowing rivers, where the particles can become rounded.
Sandstone: consists primarily of sand sized (1/16-2mm.) grains.
Mudrocks: Consists of clasts < 1/16 mm. i.e. smaller than the eye can readily distinguish. In this size range are two clast catagories: silt (1/16 - 1/256 mm.) and clay (< 1/256 mm.). If you want to make the distinction without a microscope, silt feels gritty in your mouth, clay feels smooth.
Sorting:
Sorting is the range of sedimentary grain sizes that occurs in sediment or sedimentary rock. If that range is narrow, we say the deposit is well sorted. If it is wide, we call it poorly sorted. Sorting is very revealing about methods of transport. For instance, wind-blown sediments are always very well sorted. Sediments transported by glaciers are very poorly transported. A well-sorted sandstone tends to have greater porosity than a poorly sorted sandstone because of the lack of grains small enough to fill its pores. Conglomerates tend to be poorly sorted rocks, with particles ranging from boulder size to clay size.
One last distinction - concretions: Quite often, because of local variations in groundwater chemistry, a small region (usually pea to watermelon sized) will precipitate a harder or less soluble cement than the surrounding area during cementation. Quite often, these remain intact after the surrounding rock has weathered away. They typically form when a mineral precipitates and cements sediment around a nucleus, which is often organic, such as a leaf, tooth, piece of shell or fossil. For this reason, fossil collectors commonly break open concretions in their search for fossil animal and plant specimens. Concretions are formed from mineral precipitation around some kind of nucleus. A nodule is a small, irregularly rounded knot, mass, or lump of a mineral or mineral aggregate that typically has a contrasting composition, such as a chert nodule in limestone, from the enclosing sediment or sedimentary rock. Normally, a nodule has a warty or knobby surface and exists as a discrete mass within the host strata. In general, they lack any internal structure except for the preserved remnants of original bedding or fossils. Nodules are closely related to concretions and sometimes these terms are used interchangeably.
How to Log This Earthcache
You must visit the area in order to get credit for this Earthcache. Armchair finds will be deleted without explanation (see Earthcache Guidelines). In order to claim the find, send me a message with the answers to the following questions:
1) At the posted coordinates, what is your hypothesis of the rock found here:
- How would you classify this rock – clastic, chemical, or biochemical?
- What is the basis of your conclusion?
2) For the dark, rounded pits in the rock, what do you think make up these grains? What size on the Udden-Wentworth scale are these?
3) Would you describe these grains as well-sorted or poorly sorted? Why, in what environment do you think these sedimentary rocks were formed?
4) What does your GPS receiver say is the elevation at your present location? If you do not have a GPS with you, use your phone. Many phones and several geocaching apps can tell you your present elevation.
5) (Optional) This is not required for logging this Earthcache, but it makes for a fun memory of your find. Post a picture of yourself with your rock find and /or your entourage anywhere in the area.
Ref:
https://www.geol.umd.edu/~jmerck/geol100/lectures/13.html
https://opentextbc.ca/physicalgeology2ed/chapter/6-1-clastic-sedimentary-rocks/
https://www.priweb.org/blog-post/concretions
https://en.wikipedia.org/wiki/Nodule_(geology)
https://en.wikipedia.org/wiki/Concretion