THE STORY OF OHIO'S ROCKS 



23 



plants and in some beds tree trunks and logs are well preserved. Both seed ferns and true 

 ferns have been found. In Ohio the evidence of their existence consists mainly of spores, 

 which are abundant in some beds, and of fragments of fronds and stems. The much greater 

 quantities of plant material indicate that in Mississippian time the earth was extensively 

 clothed with vegetation. Coal beds of this age in Virginia foreshadow the widespread coal 

 swamps of the Pennsylvanian. 



Upper Mississippian rocks are not present in Ohio. Rocks of this age in other parts of 

 the country, for example in western Kentucky, record the change in animals which took place 

 during this time. The sea may have covered parts of Ohio late in Mississippian time but if so, 

 the sediments which it laid down were washed away before the beginning of Pennsylvanian 

 time. The surface of the land was carved extensively during this time as shown by the earliest 

 Pennsylvanian deposits which filled hollows in the Mississippian surface. 



PENNSYLVANIAN TIME: THE COAL FORESTS. 



So far, the rocks have provided us with abundant evidence of the nature of the seas and 

 the life that inhabited them. It gave us only occasional glimpses of land life. The Pennsylvani- 

 an record is almost equally divided between the land and the sea (fig. 24). This unique situa- 

 tion is due to the peculiar state of the surface of parts of North America during this time. It 



was low and almost flat, especially east of the Miss- 

 issippi and lay very near to sea level. The relative 

 level of the sea and land changed often during Penn- 

 sylvanian time so that short invasions of the sea, 

 alternating with deposition of non-marine sediments, 

 occurred many times during the period. When the 

 lowlands were not under the sea, they were covered 

 by dense swamp forests which supplied the materials 

 for coal beds. 



— ,r— A Shale and sandstone 



Clay non-marine 



Iron ore marine 

 Limestone marine 



Coal non-marine 



Fig. 24 Diagram of a typical "Cyclothem" 



or group of rocks recording one cycle 



of submergence and emergence. 



non-marine 



The most remarkable feature of Pennsylvanian 

 rocks, in Ohio and elsewhere, is the presence of coal 

 beds, some of them very thick. This valuable fuel 

 owes its origin to the accumulation of plants under 

 swampy conditions. Time after time, between in- 

 vasions of the sea, parts of Ohio looked like the 

 great Coastal Plain swamps of today: low-lying 

 marshy areas, densely covered with trees and other 

 plants, growing right to the edge of the sea. Great 

 thicknesses of plant debris accumulated in these 

 vast swamps. Gradually, sea level rose. Salt water 

 crept into the swamps, killing the trees. The swamps 

 were buried under sand and mud whose weight com- 

 pressed the plant debris which underwent slow chem- 

 ical changes and became coal. Coal may not look much like plant material but under the mic- 

 roscope its true nature is apparent; the plant fibers and sometimes their cell structure are 

 clearly revealed. 



The beginning of coal mining in Ohio goes back to 1803. Coal-bearing rocks in 30 counties 

 of Ohio cover 12, 340 square miles and the total value of coal mined in the state in 1952 was 

 $132,000,000. 



The seas that invaded Ohio in Pennsylvanian time came from the west and south. Their 

 pattern changed many times during the period and the coal swamps were either retreating 

 before the sea or advancing to occupy its shores. The rocks laid down in Ohio are therefore 

 marine and non-marine, chiefly shales and sandstones with layers of clay and coal, and thin 

 but comparatively persistent limestones and iron ores. 



