The location:
This EarthCache is located in the centre of Newbury and asks you to examine the rock used as facing material on the external walls of a building housing a branch of a high street bank. The required information is visible from the street and there is no need to enter the building.
The EarthCache lesson:
Rudite is a general term for sedimentary rocks composed of coarse-grained fragments that are larger than sand (e.g. granules, pebbles, cobbles and boulders) set into a fine-grained matrix that holds the fragments together. These fragments, or 'clasts', can be either rounded (like in conglomerates) or angular (like in breccias). The term is also used as a synonym for psephite.
The distinguishing feature between a conglomerate and a breccia is the shape of the constituent clasts. A conglomerate contains rounded clasts, showing that the rock fragments underwent one or more erosion process (such as weathering in the air or attrition under water) before they became incorporated into the conglomerate. By contrast, a breccia contains angular clasts from faulting, folding or volcanic activity and did not undergo any mechanical changes after being formed, i.e. they became cemented into the breccia before such actions had taken place. Many examples of rudite are easily classified into conglomerate or breccia simply by examining the shape of the clasts.
Conglomerate with quartzite clasts from the Ordovician formations near Bergen, Norway. Photo © Siim Sepp.
The polymict breccia of Mosaic Canyon is produced by mud flows that travel through the canyon depositing fragments of different rocks together. Death Valley, California. Photo © Michael C. Rygel via Wikimedia Commons.
Rudites that contain clasts of different types of rock can have a very attractive appearance and are often used for decorative purposes. The size of clast can vary, indicating the degree of sorting undergone by the fragments prior to being fixed in position. A sorting chart can help determine this characteristic, as shown below (acknowledgements: British Geological Survey).
The EarthCache:
At the posted coordinates, face west and examine the stone facing of the building in front of you. Focus on the stonework making up the majority of the exterior facing and especially around the outside cash dispenser to the left-hand side of the main entrance. Do not look at the surfaces around the window frames, which are simply pebbles set into cement. In particular, observe the size, shape and colour of the clasts and then answer the following questions. Your answers should be sent to the CO via the Geocaching message service or by email.
1) by consideration of the descriptions given above, is the rock in front of you a conglomerate or breccia? Justify your conclusion.
2) what is the length of a. the smallest and b. the largest fragments you can see?
3) based upon your answer to question 2 and using the sorting chart above, how well sorted is the rudite?
4) what is the predominant colour of the rock?