What are fossil rocks?
A fossil is something preserved from a plant or animal of the dim, distant past. It may be a bone or a footprint, a layer of coal or a lump of petrified wood. in any case, ii; was first made by some living thing and rocks seem to be quite the opposite from the teeming world of living things. But there are fossil rocks and also fossil bearing rocks, which need to be explained.
The story of life on our planet is written in a diary which dates back millions of years. The clearest records are written in sedimentary rocks such as layers of shale which, year by year, add papery. thin pages of dried mud and silty sediments.
Through the ages, all sorts of living things have become trapped in these rocky pages, much as you press a leaf specimen between the pages of a book. In time, the specimens in the earth's rocky diary become fossils. The fossilized imprint of a feather or leaf, a bone or tooth may be preserved in the rock.
A lucky scientist. may find a layer of shale or sandstone teeming with these fossils. He will call his treasure trove a layer of fossil bearing rocks. If we call this precious diary a layer of fossil rocks, he could be quite wrong. But if the sedimentary layer is made of limestone, we would be quite alight. The preserved fossils would then be buried in fossil clock.
Limestone and chalk, coal and amber, petrified wood and many other minerals of the earth's crust are true fossil rocks. They are classed as rocks because they are made from the same minerals which make the rocky minerals of the earth's crust. They are classed as fossils because they were originally formed by from living plant or animal.
Chalk and limestone began to form or, tho floors of the ancient seas. Tiny creatures lived and died in the water and their shells sifted down to form chalky layers of salty mud. In time these seabeds became dry land and the piles of tiny shells hardened into layers of rock. We can call them fossil rocks because the bulk of their material was first formed by living things.
Coal was formed from the woody remains of ancient forests. It too is a fossil rock and when we find the imprint of an ancient leaf pressed between its black pages, it is also , fossil bearing rock, Petrified wood is made entirely of rocky minalalua It is a fossil rock because the pattern of the minerals is a perfect copy of the cells grown by some old, old tree.
Many fossils, it seems. are. made from stony rocks. Sometimes a fossil bone is made of stone. The original relic was replaced with a copy in stony minerals.' But not all fossils are made of rock. A lump of amber is a gob of resin preserved from some ancient pine tree. Fossil ivory is the actual tooth or tusk preserved from an ancient elephant or walrus.