The posted coordinates bring you to St Patrick's Street in the middle of Cork City Centre. You will see giant red blocks with a very unique texture to it. Choose any one to answer the questions and enjoy this urban EC on one of the four marbles of Ireland.

Clastic Rocks

You may notice that the Cork Red Marble seems to be fractured, like cracks in the walls of a building. But if you take a closer look, you can see the the different pieces of rock are of different shades of red. The material itself is a clastic rock: chunks of smaller rocks that have broken off of other rocks and resedimented and cemented together.

There are 2 types of clastic rocks: breccia and conglomerates. They are usually subdivided according to the average size of their constituent materials into pebble (fine), cobble (medium), and boulder (coarse).

Breccia and Conglomerates

The formation of breccia and conglomerates are more or less the same. Pieces/chunks of existing rock break off and are transported into a basin or depression. They are then cemented with a matrix, similar to shale and sandstone, thus these two rock types are sedimentary rocks.

One major difference between breccia and conglomerate is the shape of the clasts: breccia has angular clasts, indicating that the rocks were not transported far to round of the edges. Conglomerates have rounder and smoother clasts.

Classifications

Conglomerates can be classified in a few ways.

• The composition of the clasts. If all the clasts are the same type of rock or mineral), the rock is categorized as monomictic conglomerate. If the clasts are made up of two or more rocks or minerals, the rock is a polymictic conglomerate.
• The size of the clasts. Rock comprised of large clasts is cobble conglomerate. If the clasts are pebble-sized, the rock is called pebble conglomerate. If the clasts are small granules, the rock is called granule conglomerate.
• The amount and chemical composition of the matrix. If the clasts do not touch each other (lots of matrix), the rock is paraconglomerate. Rock in which the clasts touch each other is called orthoconglomerate.
• The environment that deposited the material. Conglomerates may form from glacial, alluvial, fluvial, deepwater marine, or shallow marine environments.

Cork Red Marble

Cork red marble is a red-lime conglomerate, interstratified with reef limestone. It is sourced mainly from Southwest Ireland and is used as a decorative stone in many Irish towns and cities. This clastic sedimentary rock can yield fossils and calcite pebbles/veining. Also present on the surface of the rock are stylolites,  a secondary (chemical) sedimentary structure consisting of a series of relatively small, alternating, interlocked, toothlike columns of stone, forming a zigzag pattern. These materials are then cemented together by hematitic stained clay, giving it it's iconic red colouring.

Questions

Send the answers to the following via my profile. Log once answers are sent. I will get back to you if there are any unsatisfactory answers.

1. Is Cork Red Marble a breccia or a conglomerate? Justify your answers.
2. Do you think the clasts have been moved around much after settling down? Explain why you think this is so.
3. Identify any stylolites or fossils. How long are the stylolites? How big and what shape are the fossils?
4. Categorize Cork Red Marble by the following:
   • Composition of clasts.
   • Size of clasts.
   • Clast-Matrix ratio.