Limestone is a sedimentary rock, composed mainly of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). The primary source of the calcite in limestone is most commonly marine organisms. These animals and plants extract calcium carbonate from seawater in order to build up their shells and stems as they grow. When these organisms die the hard parts of their bodies are broken up by waves and the pressure of material and water above them, forming layers of sediment. These layers pile up and over time if conditions are suitable they are compacted into limestone.
Limestone appears in many forms, from fine-grained smooth rock to very coarse-grained rock with many inclusions present. Shelly limestone is, as the name suggests, limestone with a high percentage of fossilised organisms embedded in it. Typically these are a large number of broken shells which are bound together with calcite. We may think of fossils as strictly bones or shells, or imprints of those which remain in stone, but it can be any preserved remains, impression or trace of any once-living thing from a past geological age. Even an ancient footprint can be considered a fossil.
At this Earthcache you can see blocks of shelly limestone, part of the Glanville Formation which was exposed while the St Kilda channel was being excavated. The Glanville Formation represents marine shelled animals which lived near shore some 125,000 years ago. At that time the climate was considerably warmer than at present and many of the fossil shells now recognisable in the Glanville limestones are those of animals presently confined to tropical and sub-tropical seas. During the last glacial episode, sea levels lowered considerably as the polar ice caps increased in area and thickness. Locally the sea retreated to the present mouths of the gulfs, so exposing the shelly sands to the influence of the atmosphere. Much of the shelly sand developed a hard laminated crust which now separates Glanville Formation sediments from those forming today, the St Kilda Formation.
To claim a find on this Earthcache:
Visit the published coordinates of this cache, where you will find some large stone blocks, one of which has an information plaque on it. Examine the blocks and email me your answers to the following:
1. Briefly describe the process by which this limestone was formed.
2. Look closely at the rocks and describe what you see. What colours are present? Can you see any grains? Is the surface smooth or uneven?
3. Find a fossil in one of the blocks and describe it. Choose an unusual one rather than one of the many flat mollusc shells. What kind of organism do you think this belonged to?
Feel free to post a photo of yourself in the area, but don't give away the answers in your photo. You may post your Found log immediately without waiting for my response. I may not contact you unless I feel your answers require clarification. If I don't receive your correct answers within 7 days of your Found log, I may delete your log. Please don't post answers to the questions above in your log.