THE STORY OF OHIO'S ROCKS 



15 



Whitewater 75' 



Liberty 30' 



=E?p?| Waynesville 95' 



^r_=rz_rl Arnheim 60' 



McMillan 75' 



=r^=^. Fairview 110' 



■^J Latonia 240' 



Trenton! 



Such accumulations of limestone are forming, for ex- 

 ample, around the Bahama Islands at the present time 

 and the climate of Ohio in Ordovician time may be 

 compared to that of these islands. Shales are clastic 

 rocks (i. e. rocks formed by the breaking up of other 

 rocks) and they are extremely fine-grained. When 

 rocks are disintegrated by weathering, the resulting 

 particles are transported by streams to the sea, then 

 by currents in the sea, until the currents can no longer 

 carry them. When that happens, the particles settle 

 down to the bottom and form sediments later consolid- 

 ated into rock. Swift streams can carry particles of 

 all sizes, even large boulders. As their energy de- 

 creases, the streams can no longer carry the larger 

 particles, i.e. the boulders and pebbles, so these are 

 dropped and form gravels which are consolidated into 

 conglomerate. Next to be dropped are the sand-size 

 particles which eventually form sandstones. Last to 

 be dropped are the finest particles, which form muds 

 and eventually harden into shales. 



Some parts of the rock are dissolved in the water, 

 for example, calcium carbonate or lime. Materials in 

 solution are carried much farther than undissolved 

 particles; therefore, limy muds are deposited much 

 farther out at sea than shales. As we look at a section 

 of Ordovician rocks in Ohio, we see limestones alter- 

 nating with shales. In some places they are of great 

 thickness, in others very thin beds of the two kinds of 

 rock, alternating in thick sequences. 



From these deposits, we can surmise that during 

 lime deposition the sea was clear and only minute flakes 

 of lime were settling to the bottom, far away from shore. 

 During shale deposition, either the shore was closer to 

 the area of the section or else the land was fairly high, 

 so that muds and clays were being washed into the sea. 

 When we encounter a bed of sandstone in a section (sand- 

 stone is rare in the Ordovician of Ohio) we must conclude 

 that sand was being deposited near shore and that the 

 water above it was muddy, but that currents could still 

 carry away the mud to deposit it elsewhere. 



Cynthiana 50' 



Fig. 12 Ordovician rocks of Ohio 



Under the waters of the Ordovician sea marine 

 life teemed in abundance that can scarcely be matched 

 in the world today (fig. 13). Some of these marine crea- 

 tures would look familar; there were seaweeds, clams, 

 and snails and lobster -like animals in great numbers, built on patterns very similar to those of 

 present-day seas. But there were also some strange animals - strange because they have 

 long been extinct - and the marine biologist would miss some of the more familiar creatures 

 of the present-day seas. 



Among the more peculiar forms of life in the Ordovician seas were the graptolites (see 

 Chapter 3) and large relatives of the squids and devilfish, the nautiloid cephalopods, which 

 were abundant in the Ordovician seas. Most of the nautiloids had straight shells. Among 

 the largest of the straight nautiloids in Ohio was one form which attained a length of 3 feet 

 but even it was dwarfed by a giant of the same group, found in the Ordovician rocks of 

 Montreal, Canada which reached a length of 15 feet and a diameter of one foot. 



