Science questions answered: stone shells

A few weeks ago, I got an email from a reader with a question for me to answer. This is what they said:

 “When I was in Olympia in Greece looking at the ruins I noticed some of the stone blocks had loads and loads of shells embedded in them. Now, I understand that it is common to see shells inland because either they were brought there or because once the land was underwater. What I don’t understand is how shells *became* rock all together like that. Can you explain?”

I didn’t know the answer to this question, so I asked someone who knows about rocks. Here is their answer, which I think is very interesting:

“Sedimentary rock is made up of accumulated sediment, which is, over millions of years, converted to stone through pressure. Sediment might be the remains of animals, or crushed up pieces of other rock, or fine particles of mud and silt. Limestone is a sedimentary rock which is formed of the skeletal remains of marine organisms. Sometimes they are crushed into small particles, sometimes they are recognisable as the animals they once were. The types, completeness and concentration of animal skeletons that you find in different strata of limestone depend on when (which geological period) and where they were deposited (the ‘depositional environment’). By ‘depositional environment’ I mean not just ‘the sea’, but where in the sea.

Heat energy from the sun warms the earth unevenly which causes air to move around and create wind. The wind then blows across the surface of the ocean, creating waves, and waves contain a lot of energy. As you will know, oceans contain a wide variety of different conditions – from shallow beaches to deep ocean troughs. These different conditions contain different levels of energy – deep oceans are very low energy, because waves do not penetrate the oceans very far, whereas shallow beaches are very high energy because all of the energy in the waves is contained within a small amount of water. High-energy environments can carry much larger particles, like whole shells, and can move them around, sorting them in an effect called winnowing. This can cause an accumulation of similar sized shells in an area, many more than you would expect if the shells were simply dropping to the ocean floor and not being moved. When these deposits become limestone the rock is characterised by a large number of closely packed shells. So, I believe that the rocks your reader saw in Greece were deposited in a high energy environment such as a shallow beach or offshore sandbank where the energy was sufficient to move larger particles and sort them together, before they were covered with other sediment and began the process of becoming limestone.”

I hope you found this post interesting, and please post a comment if you would like me to answer a question!