neatl)- preser\'ed shrimp, the fossil remains of a creature 

 that lived here more than a quarter of a billion years ago, 

 a specimen unseen by human eyes imtil you picked it up. 

 Forget the slippery hills, the prickle-bushes, the beating sun ! 

 Forget the possible thunderstorm; you're out for fossils! 



The strip mine was not actually dug for the purpose of 

 turning up fossils. From the point of view of the operator 

 (a curious view, perhaps) the fossils are a by-product and 

 coal is what they're after. Fifty to a himdred feet below the 

 flat farmland lies one of the most extensive beds of coal on 

 this continent. Known here as the Wilmington Coal, it is 

 the Colchester Coal of western Illinois, the Lower Kittan- 

 ning Coal of Pennsylvania, and has other names where it is 

 mined in Oklahoma, Missouri, Kentucky, Michigan and 

 other states. 



Coal, as is well known, is made up of the carbonized re- 

 mains of plants. Plants in such numbers as to make up this 

 vast bed mean a forest of uncommon size. Study of the 

 Pennsylvanian rocks and coals by many geologists in the 

 century past enables us to visualize this forest of the Wil- 

 mington Coal. Giant tree-sized ferns and horsetails and 

 their allies, shallow-rooted, quickly growing, continually 

 falling and accumulating on the sodden ground, lived in a 

 vast flat swamp. Only under water can the fallen trees go 

 through the proper chemical changes to become coal. 

 Broad though that swamp forest was, it continually shifted 

 its position, forming a broad band, probably hundreds of 

 miles across, between an inland sea to the south and west 

 and the modest uplands to the north and east. Trees on 

 those uplands, falling on drier ground, decayed without 

 trace while the trees in the great swamp built up the layer 

 of substance that was almost coal. Sea level rose (or the 

 crust of the continent sank) ; gently the sea advanced across 

 Oklahoma, Missouri and Illinois. Now the former swainp 

 forest was buried under sea-floor muds; the broad belt of 

 swamp moved on ahead into what had been the uplands. 

 For a while, when the shoreline lay here near Wilmington, 

 the advance of the sea was halted. 



This was the time when the fossils that we find were liv- 

 ing. For you are walking now on that former shore. But 

 how shall we define that shore? It was no sandy strand with 

 land on one side and sea on the other. Back into the swamp 

 forest ran countless intricate inlets, bays, bayous and chan- 

 nels; far out to sea stretched a complex of islands, bars, 

 peninsulas and shallows. Mud, carried through the swamp 

 by sluggish streams, poured into the edge of the sea, build- 



ing a delta. Like all deltas, it was chiefly under water. In 

 lime, before the sea resumed its march across the low and 

 swampy land, the delta muds-built up to a hundred feet in 

 thickness. Now, this seems utterly improbable in water that 

 we have solenmly declared to be shallow. The answer lies 

 in the behavior of the earth's crust. Under the center of 

 Illinois during this Pennsylvanian period the crust sagged 

 more than elsewhere. While the shoreline tarried at the 

 strip mines, the inland sea floor slowly dropped at about the 

 same rate as the mud accumulated, and the water remained 

 shallow. 



Animals of many kinds lived on the flanks of the growing 

 delta, drawn by the nourishing water near the swamp. 

 Worms and snails, shrimps and clams, chitons and sea- 

 cucumbers moved about on the mud surface, some in great 

 numbers. Through the murky water swam Tully Monsters, 

 sharks, bony fishes, shrimps, scallops and many others. The 

 water draining from the land carried along fronds and pin- 

 nules, spores, stems and seeds; insects, thousand-leggers, 

 spiders and little amphibians drifted with them out onto 

 the delta. 



All of these and more, falling to the sea floor, were 

 quickly covered by the raining mud — so quickly on the 

 whole that they had no time to decay before they were en- 

 cased forever in the firm sediment. Before decay could 

 attack, each little fossil-to-be was locked into place by a 

 halo of iron mineral that soaked and hardened the shale it 

 lay in. Teeth and bones remained unchanged as time went 

 on; shells changed from one limy mineral to another or dis- 

 solved, leaving a perfect mold. 



But what of the soft tissues of the animals? They broke 

 down chemically after having left their impress upon the 

 rock. All that remains today for the collector is the impres- 

 sion, plus some invisible amino acids and other organic com- 

 pounds that have soaked into the rock. Except in these strip 

 mines, it is most extraordinary to find even an impression 

 of a soft-bodied animal such as a Tully Monster or a worm 

 — or a jellyfish. Many of these impressions, exposed on the 

 broken equator of a split concretion, have almost no relief — 

 no ups and downs. It is the organic leftovers that finally 

 make them visible. When a concretion has been cracked by 

 sun and frost, the inside surface is exposed to two powerful 

 oxidizing agents, sun and air. Between them, they make 

 the ironstone turn red, just as the outside of the concretion 

 did before. But where the amino acids had soaked the rock, 

 on the impression of the fossil, they take up the oxygen be- 

 fore the iron gets it, and the impression remains pale. 



Drawings, by the author, of five different specimens of Octomedusa pieckorum. Invertebrate fossils may be preserved at 

 any angle, and a series such as this provides a great deal of information about the animal. 



OCTOBER Pages 



