14 



OHIO FOSSILS 



CAMBRIAN TIME: THE AGE OF TRILOBITES. 



Fig. 10 Late Cambrian lands and seas 



Some 500 million years ago, the area that is now 

 Ohio was invaded by the Cambrian sea (fig. 10). We 

 know little of the history of this marine invasion of the 

 state for again, its sediments are buried under younger 

 rocks. Only in a few deep borings have Cambrian rocks 

 been penetrated. 



At the base of the Cambrian in Ohio is a deposit 

 of almost pure sandstone some 135 feet thick, overlain 

 by about 600 feet of dolomitic sandstone and sandy dolo- 

 mite. 



Elsewhere in the world, Cambrian rocks have a 

 record of abundant life and we can reasonably suppose 

 that the Ohio area was also thickly populated with marine 

 animals. 



For much of Cambrian time Ohio was out of water. 

 It was only towards the end of Cambrian time, when the 

 seas reached their widest extent, that Ohio was covered 

 by their waters. The sea drained from most of North 

 America towards the end of the Cambrian period but inmost 

 places the retreat was a comparatively brief one, 

 followed by a re -invasion. 



ORDOVICIAN TIME: THE AGE OF BRYOZOAN REEFS. 



Fig. 11 Middle Ordovician lands and seas 



If we could be privileged to travel back into time to 

 the middle of the Ordovician, and either cruise over the 

 inland seas of America (fig. 11) or examine them from 

 the air, we would be struck by the differences between 

 the appearance of Ohio in this far -distant time and its 

 present aspect. We could find little to orient ourselves 

 by, for the entire state was under the sea. The sites of 

 our major cities would be under water and the familiar 

 landmarks, such as the Ohio River and the Great Lakes 

 would still be far in the future. Instead of the land area 

 of the present day, we would see nothing but a mighty 

 arm of the ocean, stretching from the present region of 

 the Gulf of St. Lawrence far to the south and west, joined 

 on the north by two other great inland seas reaching from 

 Ontario, Minnesota, and North Dakota to the Arctic Ocean. 

 The sea was fairly deep - about like those around the mar- 

 gins of the continents at the present time - over eastern 

 Ohio. It became shallower in the western third of the state. 

 At times during the Ordovician, the western third of the 

 state emerged above the sea as low muddy islands but 

 never as high, mountainous land. 



The nature of the Ordovician rocks of Ohio gives us some idea of the kind of environment 

 in which they were laid down (fig. 12). The majority of our Ordovician rocks are limestones and 

 shales and these accumulate under very special conditions. Limestones are precipitated out 

 of sea water far from shore or near low-lying lands, generally in mild or tropical climates. 



