10 



OHIO FOSSILS 



corals and sea shells lie below those with ferns and tree trunks and others above them, (see 

 figs. 6a and 6b). 



Fig. 6b Continental environment 



Corals live nowadays in the sea and are not found in lakes and rivers. Ferns and trees 

 obviously live on land or at most in swampy places. With these facts in mind, we are ready to 

 take the next step which is to conclude that wherever we find corals and their associates in place, 

 undisturbed in the rock, that spot was once covered by the sea. It is nothing short of staggering 

 to realize what this means: the whole of Ohio was .covered not only once, but several times by 

 the sea and where now our broad highways run there once flourished great coral reefs teeming 

 with marine life. Ohio was only part of a great inland sea that extended at times from the Gulf 

 of Mexico to the Arctic. The expanse of water was broken only by scattered islands and by great 

 land masses to the north in Canada and to the east in the region of the Appalachian Mountains. 



In later times, as the fossils in younger rocks indicate, the sea withdrew from Ohio and 

 the state was a vast swamp covered with abundant vegetation, some of which formed the coal beds 

 which we find between layers of rocks carrying fern leaves and tree trunks. 



Fossils can help us further in deciphering the events of the past. If we collect them care- 

 fully and note their position in the rocks we are soon struck by the fact that different layers of 

 rock carry different kinds of fossils. Again using the law of superposition, we can see that the 

 rocks record the changes in life through the ages. We can thus follow the changing pattern of life 

 in the many different seas that covered Ohio in the past. 



The rocks and their fossils can thus give us the 

 order in which geologic events took place. For dates 

 in years for these events, we must turn to other 

 methods and other parts of the world. In many places 

 on the earth the surface has been broken by great 

 upwellings of molten rock from the depths below; the 

 process in still going on in volcanic regions. As the 

 molten rocks cool, either at the surface or in the 

 channels through which they came to the surface, the 

 minerals in them crystallize, separating from the 

 molten mass according to their respective melting 

 points and forming igneous rocks. It is obvious that 

 wherever we find igneous rocks cutting through other 

 rocks, the igneous rocks are younger than the rocks 

 that they cut. If, for example, igneous rocks cut 

 sedimentary rocks of Ordovician age, the material 

 that formed the igneous rocks welled up through the 

 sedimentary ones after the Ordovician rocks were 

 formed (see fig. 7). Likewise, if these igneous rocks 



are overlain by Silurian rocks, we can say that they were intruded after the Ordovician rocks 



were laid down and before the Silurian ones were formed. 



Fig. 7 Igneous intrusion cutting 

 sedimentary rock layers 



