GEORGE BERNARD / SCIENCE PHOTO LIBRARY GEORGE BERNARD / SCIENCE PHOTO LIBRARY
Section of the Earth. Earth, showing its internal structure, from The Botanic Garden (1789-92), a poem on science and nature by Erasmus Darwin. The scientific study of the Earth's rock strata did not begin until the late 18th century, when it was discovered that certain types of rock were layered over each other in a definite order. Limestone was often deeply buried, for example, while chalk was always found near the surface. The origin of such patterns, however, was not discovered until the late 20th century, when the theories of plate tectonics and continental drift revolutionised the science of geology.
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