THE STORY OF OHIO'S ROCKS 



The geologist cannot record events year by year, as the historian does, except in very 

 special circumstances. Still, a number of kinds of evidence can be used to divide geologic 

 time into broad segments and to fit events into a co-ordinated time-table which applies to the 

 entire earth. The geologic time-table (see fig. 4) is divided into eras, periods, and epochs 

 which can be dated approximately by methods which we shall examine in some detail. These 

 methods are based on simple concepts. 



The first and most important of these concepts is the "law of superposition. " It states 

 simply that older rocks are laid down before younger ones. Wherever layered rocks are exposed, 

 for example in a river valley or in road cuts, we can be certain that the layers were laid down 

 as mud or sand one on top of the other and that the rocks at the bottom are older than those at 

 the top. This simple rule is one of the bases of all historical geology; the events that caused the 

 rocks to be laid down can be arranged in their correct order only if we assume that the law of 

 superposition is valid. The law is subject to a few exceptions. For example when rocks are 

 intensely folded they may be overturned. But these exceptions do not apply in Ohio where the 

 rocks are either flat-lying or only gently inclined. 



West 



— Cambrian- and-Ordo^r) 



East 



Fig. 5 Geological section across Ohio 



With this in mind we could start in the Cincinnati region (see fig. 5) and see the nature 

 of Ordovician limestones and shales. Traveling northward and eastward, we would notice, by 

 the time we reached Dayton, Xenia, or Wilmington, that the rocks exposed in the quarries and 

 streams were different. We would have crossed the boundary between the Ordovician and 

 Silurian rocks which is usually termed the Ordovician-Silurian contact. Moreover, in good 

 exposures, we could see that the Silurian rocks lie over the Ordovician ones. At New Holland, 

 West Jefferson, and east of Tiffin, we would cross the Silurian-Devonian boundary and likewise 

 see that the Devonian rocks lie over the Silurian ones. Similarly, we would go from Devonian 

 to Mississippian, Pennsylvanian, and Permian rocks. In the northern and western parts of the 

 state the rocks of the Paleozoic systems are covered by glacial deposits. 



If we examine these rocks in detail, we find that many of them bear abundant fossils, 

 which are not merely strangely shaped rocks but the remains of animals that once lived and 

 died where we find them. Knowledge of the nature and significance of fossils gives us an idea 

 of events that were taking place in Ohio as the rocks accumulated. 



Fig. 6a Marine environment 



We can recognize among the fossils of Ohio abundant remains of corals, sea shells, ferns 

 and tree trunks. Let us take a step further and see what these mean. First, it will soon be 

 obvious that some rocks contain corals and sea shells but no fern leaves and tree trunks. Like- 

 wise, the rocks with ferns and tree trunks contain no corals or sea shells. The fossil collector 

 will soon learn that if he wants ferns and tree trunks he must look in certain beds and that the 

 corals and sea shells are abundant only in others. Also he will find that some of the rocks with 



