18 



OHIO FOSSILS 



Brine *• 



■^N 



mm^b^sS 



Fresh water 



< y 



Rock salt 



Late in Silurian time, the sea in the northern 

 part of the state was of a peculiar character, shallow 

 and sometimes cut off from the main body of the sea. 

 Great accumulations of salt were deposited as the sea 

 water evaporated. They were preserved when muds 

 covered them over. The beds of rock salt underlie 

 an area of 9,000 square miles in northeastern Ohio. 

 They vary in thickness from 2 feet to nearly 50 feet 

 and they are separated from each other by beds of 

 dolomite and shale. The uppermost salt bed in the 

 Cleveland region is under about 1, 300 feet of younger 

 rocks; to the south and east it may be as far down as 

 4, 767 feet. Salt is extracted from these beds by 

 pumping fresh water into wells, where it dissolves 

 the salt, and pumping out the brines which are then 

 evaporated or used directly by Ohio's great chemical 

 industry (fig. 17). The salt industry in Ohio goes 

 back to 1889. For a more complete account and 

 references, see J. F. Pepper (1947). Salt is combined 

 with the Silurian limestones and dolomites to manu- 

 facture soda ash, caustic soda, chlorine, and many 

 other products. 



The salt basins were cut off from the main body 

 of the sea as it slowly withdrew from the continent 

 in late Silurian time. There was complete withdrawal 

 of the sea from Ohio between latest Silurian and earli- 

 est Devonian times. 



The marine animals which lived in the Silurian sea 

 were very much like those of the Ordovician. Many genera 

 and some species persisted from Ordovician to Silurian 

 time and were accompanied by others which had recently 

 evolved. Some of the latter are characteristic of Silurian 

 rocks and are described later (Chapter 5). 



Fig. 17 Diagram of a salt well 



The most striking feature of the Silurian seas 

 was the abundance of coral reefs. Coral reefs had 

 existed in Ordovician time, but they did not become 

 widespread until the Silurian. Our reefs are the south- 

 ern extension of a great reef that bordered the Michigan Basin and extended from western 

 Ontario into Michigan, Wisconsin, Illinois, Indiana, and Ohio. A variety of animals and some 

 lime -secreting algae helped build up the reefs. The great accumulation of animals with soft 

 tissues probably was the source of the oil which is now found stored in the porous structure of 

 3 the reefs. No Silurian reef production 



is yet on record for Ohio, but the 

 Clinton sand of this age is the great gas- 

 bearing stratum of the state. Reef pro- 

 duction is still a possibility where the 

 Silurian rocks are buried deep under the 

 surface. 



The Silurian seas swarmed with 

 nautiloid cephalopods (see fig. 13 for 

 typical nautiloids), both straight and 

 coiled, some of large size, others highly 

 ornamented. Along the margins of the 

 late Silurian seas, perhaps in brackish- 

 water pools and lagoons, the eurypterids 



Fig. 18 Eurypterid 



