Walking past this building one day, my eyes were drawn to this beautiful specimen of a fossil. Most Earthcaches about limestone particularly focus on Carboniferous Limestone, which is black in colour and can be found throughout towns, villages and cities in Ireland. This stone here is a fossiliferous limestone from the Cretaceous period - Cretaceous Limestone.

Earthcache Lesson: Formation and Classification of Geological Features

Limestone is a Sedimentary Rock

Limestone is a sedimentary rock, meaning it is formed layer by layer as the main components, usually calcite or aragonite that are crystal forms of calcium carbonate (CaCO₃) deposits over millions of years and eventually cements together. Calcium carbonate is also the main component of pearls and shells from clams, snails and even eggs. 

Here in Ireland, the majority of the limestone is formed during 2 distinct geological time scales - Cretaceous and Carboniferous. Cretaceous limestone is lighter in color whereas Carboniferous limestone is grey-black. A pure form of limestone commonly found from the Cretaceous is called chalk.

Fossils

One very interesting feature found in sedimentary rocks are the presence of fossils. A fossil is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood, oil, coal, and DNA remnants.

Image depicting the formation of a fossil. Notice how it is formed within a sedimentary rock.

There are 6 ways that fossils can be formed:

• Unaltered preservation (like insects or plant parts trapped in amber, a hardened form of tree sap)
• Permineralization (in which rock-like minerals seep in slowly and replace the original organic tissues with silica, calcite or pyrite, forming a rock-like fossil - can preserve hard and soft parts - most bone and wood fossils are permineralized)
• Replacement (An organism's hard parts dissolve and are replaced by other minerals, like calcite, silica, pyrite, or iron)
• Carbonization (in which only the carbon remains in the specimen - other elements, like hydrogen, oxygen, and nitrogen are removed)
• Recrystalization (hard parts either revert to more stable minerals or small crystals turn into larger crystals)
• Authigenic preservation (a process which leaves a negative impression, or indent, of an organism in rock after the organism itself has deteriorated).

There are 4 types of fossils:

• Mold fossils (a fossilized impression made in the substrate - a negative image of the organism)
• Cast fossils (formed when a mold is filled in)
• Trace fossils/Ichnofossils (fossilized nests, gastroliths, burrows, footprints, etc.)
• True form fossils (fossils of the actual animal or animal part).

Logging Tasks

Send the answers to the following questions via my profile and log immediately. I will get back to you if there are any unsatisfactory answers.

Specimen found here: 

1. You will see a massive fossil specimen here. Describe the texture, colouration, and shape of this fossil.
2. What is the method of fossilisation here?
3. What is the type of fossil?