This EarthCache, takes you to the entrance way to the Refuge Assurance Building on Church St, Sheffield. We are here to look at the flooring, which is pubically accessible.
We are here to learn about conglomerate and breccia.
Breccia
This is the name given to a type of sedimentary rock, which are formed by the collection toegther of a large number of angular fragments, with the space between these filled by similar fragments or mineral cement. Breccias get formed when host rocks break, and their debris are not transported to any far off place. This means that these rocks form when original rocks break and re-accumulate to make pieces that are angular in texture. A high energy flash flood is an example of the environment which produces breccia, as there is not enough time for sediment to be sorted and then laid down.
Conglomerate
This is the name given to a coarse grained sedimentary rock , which is composed of rounded or subangular stones, which in geological terms are known as clasts.
Breccia can be formed by a variety of ways :
Consolidation - such as the collection of material at the bottom of steep slopes.
Cataclastic -during the process of faulting.
Volcanic - composed of ash and chunks of lava.
Hydrothermal - caused by fluid fracturing the rock mass.
Impact - from when a meteor hits the ground, it fractures the rock at the impact site.
Classification :
Matrix supported - where the clasts do not connect with other, and the matrix surrounds each clast.
Clast supported - where the clasts touch other, and the matrix fills the holes.
Clast size -
The clasts which make up the breccia, can be described in the terms of their size:
Fine (2 - 6mm).
Medium (6 - 20mm).
Coarse (20 - 60mm).
Very coarse (> 60mm).
If all the clasts are mostly the same size, it can be said that the breccia is well sorted, if there is a mixture of clast sizes, then the breccia is poorly sorted.
Breccia is basically made up of a collection of rocks, thought his can also be described, as if there is more than one type of rock present, then it is polymictic, and if there is just one type of rock, it is monomictic.
This being an EarthCache, in order to log it, I ask that you complete the below tasks. Please send the answers to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile.
1. Is the Breccia here monomictic or polymictic? What is the rationale for your answer?
2. Please define the predominant clast size here, what is it?
3. Is the Breccia matrix supported or clast supported?
4. Look at the Breccia here, which has been cut into square tiles by the mason. There is a Breccia tile, which is 3rd in from the right, and 3 tiles in, please describe the clast here, in terms of shape, size (diameter) and colour.
5. Look at the Breccia here, which has been cut into square tiles by the mason. There is a Breccia tile, which is 4th in from the left, and 2 tiles in, please describe the clasts here, in terms of shape, size (diameter) and colour.