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Fortrose Cathedral was built in 1250 A.D., and all though only a small part of the original building remains , it is still an impressive sight. Two sections, the chapter house and the nave south aisle, still stand . The outline of the remainder of the cathedral complex was revealed by excavations in 1873. It is constructed of red sandstone from Arkendeith Quarry above Avoch. In 1572 Lord Ruthven was granted permission to use lead from the roof, and Oliver Cromwell used stone from the Cathedral to build a new fort in Inverness. It stands within four walls, surrounded by lawns.
Rocks can be divided into three main groups, sedimentary, igneous and metamorphic.
Most sandstone is composed of tiny grains of quartz and/or feldspar because these are the most common minerals in the Earth's crust.The size of the particles can be subdivided into very fine (0.0625 to 0.125mm), fine (0.125 to 0.25 mm), medium (0.25 to 0.50 mm), coarse (0.50 to 1.00 mm), and very Coarse (1.00 to 2.00 mm). Like sand, sandstone may be any color, but the most common colors are: tan, brown, yellow, red, grey, pink, white, and black. Since sandstone beds often form highly visible cliffs and other topographic features, certain colors of sandstone have been strongly identified with certain regions.
A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.
The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form.
The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation.
These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.
Mineral hardness is also important in sedimentary rocks. Harder minerals tend to be able to travel longer distances down river systems. Quartz can often undergo several cycles of erosion, transportation and lithification ( change of sediments to rock). Zircons are persistent minerals in the environment and can often tell geologists the types of rock that were the original source rock for metamorphic or sedimentary rocks.
Weathering
Although quartz sandstones are not especially susceptible to solution damage from acid rain, they are quite prone to physical disruption by salts, which crystallise within their pores. These are often a combination of marine salts such as sodium chloride, complex salts contained within rising groundwater and pollution derived salts, particularly calcium sulphate (gypsum).
When salts crystallise within the stone, repetition of expansion and contraction can lead to features such as granular disaggregation (separation of grains), scaling and multiple flaking.In areas that are sheltered from rain, accumulations of gypsum on the surface can incorporate soot particles to form unsightly and potentially damaging black crusts. Over time, moisture movement corrodes metal fixings that can expand and fracture the stone.Moisture also mobilises iron from within the stone and concentrates it at the surface. This forms a hard crust, but if this is breached, the stone behind is rapidly hollowed out to form honeycombs and larger cavernous hollows.
To claim this earthcache, please send the answers to the questions below to us by email / message centre - do not post in your online log.Your log may be deleted if this criteria is not met. Educational guidelines for Earthcaches are set by Geocaching.com and GeoSociety.org (Earthcache) and have to be adhered to.
1 ) The sandstone is quite badly weathered in places along the walls, is there a particular wall ( N,S,E or W ) that shows more weathering then the others ? If yes, give reasons to why this might be ~ if not,why do you think this is ?
2) Based on the information above and your observations, what type of weathering do you think was at work here?
3 ) Red is the main colour here, what do you think has caused this colour ? Google may help.
4 ) Looking very closely at one of the weathered stones, can you see any of the individual grains and which size category would you describe them as being or do they differ ?
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