THE STORY OF OHIO'S ROCKS 13 



CLASSIFICATION OF OHIO ROCKS. Fossil collectors should have some idea of how rocks 

 are classified and the significance of this classification with regard to fossils. Rocks have 

 been subdivided according to their nature and relative age, into systems, series, groups, 

 formations, members, and beds. A general classification of rocks is given in fig. 4. Examin- 

 ation of this table will show that only Paleozoic and Cenozoic rocks are found in Ohio and that 

 the Cenozoic is represented only by Quaternary rocks, the very youngest of the Cenozoic rocks. 

 The geologic map of Ohio (fig. 8) shows the distribution of rocks by systems in Ohio. A more 

 detailed geologic map of Ohio on a larger scale can be obtained from the Ohio Division of 

 Geological Survey (price $1.00). 



To indicate the relative age of rocks and the fossils in them, the same terms are used 

 as those for identifying the rocks themselves. For example, rocks of the Ordovician system are 

 said to be of Ordovician age and they were laid down during the Ordovician period. If fossils 

 come from a particular subdivision of the Ordovician system, their age is given with relation to 

 the subdivision. For example, fossils from the Richmond group are of Richmond age, and 

 fossils from the McMicken formation are of McMicken age. Figure 4 should be consulted for 

 the relationships of time and rock units in Ohio. 



A quick way of finding out the ages of all the rocks exposed in a particular county is to 

 consult figure 9 which lists the counties of Ohio alphabetically and indicates the systems 

 represented in each county. 



PRE -CAMBRIAN TIME: THE BEGINNING 



The earliest history of the state is shrouded in mystery. Rocks from the very dawn of 

 the earth's history are present under all of Ohio but they are everywhere concealed by younger 

 rocks. Judging from pre-Cambrian rocks elsewhere - in Canada, for instance - pre-Cambrian 

 time was very long for its record starts nearly two billion years ago. During that time, sedi- 

 ments accumulated in the seas that invaded the continents many times; there were other times 

 when the crust of the earth was rent by great gashes through which molten rock poured out, 

 covering the land with sheets of lava. These dislocations of the crust were accompanied by 

 compression of blocks of the earth's surface which were folded into huge wrinkles. In making 

 its way to the surface, the molten material from the earth's interior flowed into and through 

 the sediments, searing and baking them and altering their composition. These altered rocks 

 are called "metamorphic, " that is, rocks that have been changed. 



What little we know of the pre-Cambrian history of Ohio is learned from the records of 

 deep wells. Six such wells have been drilled through the sedimentary rocks into the pre-Cambrian 

 (Stout and Lamey, 1940). Conditions in Ohio were probably much the same as elsewhere during 

 pre-Cambrian time for wherever the driller's bit brings up rocks older than Cambrian, they are 

 like the pre-Cambrian rocks exposed elsewhere at the surface, for example, in Canada or the 

 Adirondacks. 



A considerable amount of time must have elapsed between the laying down of the pre- 

 Cambrian rocks of Ohio and the beginning of Cambrian time, for everywhere these rocks were 

 planed to an almost even surface before the invasion of the Cambrian seas. Great folds, as large 

 as some of the mountain chains of the present day, were worn down to relatively flat surfaces, 

 not by some mysterious forces but by the slow, almost imperceptible action of running water 

 carrying off and dissolving the rocks from the mountains and filling in the valleys and the 

 bordering seas with sediments. When the Cambrian seas slowly spread over the land, they cov- 

 ered a low, slightly rolling plain and deposited their sediments upon it. 



No evidence of pre-Cambrian life has been found in Ohio but elsewhere on this continent 

 obscure traces of the first life on earth have been recovered. They consist of lime -depositing 

 colonial algae similar to some still living in the seas, and obscure and doubtful remains of 

 lower invertebrates. 



