|MadSci Network: Earth Sciences|
Dear Georgina, Your question places you in excellent company. Scholars in Classical Greece and Rome observed the petrified remains of seashells in rocks far from the sea. In Italy in 1484 Leonardo da Vinci was engineer on a project to build a canal for the Duke of Milan, and the diggings uncovered many fossil shells. Leonardo reasoned that they were the remains of sea creatures that had once been living. We know now, of course, that this is correct. All fossils, both of sea creatures and of land animals (like dinosaurs) and plants, are the remains of once-living animals and plants. The seashellrock you mention will be what geologists call a sedimentary rock, that is, a rock formed from sand or mud deposited often in the sea, along shorelines or in lakes. If itís made of sand it will be a sandstone, and if its made of limey mud it will be a limestone. The shells will be fossils, and geologists would call the rock a fossiliferous sandstone or fossiliferous limestone. Two things must happen to put sandstones or limestones containing fossil shells in highland areas. The first thing that must happen is that shells must be buried in sand and mud that later becomes rock. This can happen in the ocean, and also in freshwater lakes and lagoons. Just as the seashells you find on the shore today were formed by creatures living on the seabed or along the beach, so fossil shells were originally made by creatures that lived in the sea or in lakes. When they died their shells became buried in the sand or mud on the bottom, and over immense periods of time the sand and mud were buried more and more deeply and compressed until they became rock. Thus the shells became fossils. The second thing that has to happen is that the rocks containing the fossil shells must be lifted up to form mountains. To understand how this happens you need to understand how the crust of the Earth behaves. The outer part of the Earth is called the crust, and it is mostly between about 8 and 50 kilometers thick. This is very thin compared to the size of the Earth, and you can compare the Earthís crust to an eggshell. What is more, the crust is broken up into fragments (called plates) which slowly move around and jostle against each other. The energy that makes them move is heat from deep within the Earth. It is hard to imagine how rocks can flow, but the Earth is so hot inside and the pressure is so great that over many millions of years the rock beneath the crust actually flows like hot toffee, although in very, very slow motion! As the interior of the Earth flows it moves the crustal plates sitting on top. When two crustal plates smash together the rock between them becomes squeezed and lifted up to form mountains. It can take several million years for mountains to form in this way. But you might be able to experience the uplift of rocks if you are lucky (or unlucky, depending on your point of view), because rocks are lifted up during earthquakes. When crustal plates smash together the pressure between them is released as earthquakes, when the pressure squeezing on the rocks of the crust becomes so great that the rocks break and move suddenly against each other. A single earthquake can uplift a piece of the crust by several meters, and it only takes time and many earthquakes to uplift rocks by thousands of meters to form mountains. Of course, if the rocks being uplifted contain fossil shells, then eventually you will get fossils high up in mountains. I am interested to learn from you that rock full of fossil shells forms the highest land in New Zealand. I live in England, and there are many examples here also of hills and mountains made of rocks full of fossils. I hope this answers your question. Best wishes, David Scarboro
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