George, Jr. and I began collecting fossil nodules in the Wilmington [Illinois] area in the Fall of 1937.  It was no more than a change from indoor to outdor life, but as we progressed into the Spring of 1938, we began to realize that we were embarked upon a major venture in the field of natural science.  My work in the Mazon Creek deposits about twenty-five years before had failed to arouse in me any particular interest, but although the Wilmington area contained fossil nodules similar to those at Mazon Creek, it became increasingly apparent that we were seeing and finding something entirely different.  The nodules, method of collecting, degree of preservation, and many species were not entirely the same. 

By August 1st, 1938, we had accumulated more than a hundred thousand specimens, at which time George, Jr. left me to engage in business.  I donated small collections to various scientific institutions, but the finest and rarest specimens were disposed of to the Illinois State Museum, Springfield, Illinois.  From August, 1938 I continued field work alone until April 1, 1947 when I was appointed Curator of Fossil Plants at the Chicago Natural History Museum.

The Wilmington area is merely an eastern extension of the world-famous deposits at Mazon Creek, Grundy County, Illinois.  However, I have used the name Wilmington because the deposit is [so] distinctive, and because my collections were made largely in Wilimington Township, Will County, several miles west of Wilmington, the town.   Others refer to the deposit by the name Mazon [Creek], or by Braidwood, or by Coal City, two neighboring towns.

The flora and fauna of the Mazon [Creek] and Wilmington deposits occur in all parts of the world where bituminous coal is mined, but their manner of preservation is unique, rarely occurring elsewhere, and being confined closely to the Mazon [Creek] and Wilmington areas.  The fossils are preserved in flattened pebble-like concretions composed of clay silt hardened by iron and sulfur.  The contained fossil represents a plane of weakness, so that when the nodule is placed on edge [on a hard, massive surface] and split open, each half has an impression of the fossil within.  The fossils generally determine the nodule's shape, which often resembles a tailor-made fit, with a minimum of stone matrix.  This is of great convenience to the collector, who is spared the transporting of heavy masses of shale, limestone, or sandstone, such as he encounters in other localities.  There is no need for hacking out and carrying slabs and chunks.  The collector merely strolls over the waste heaps, picking up nodules and splitting them open until he has a pailful of from fifty to one hundred fifty specimens.  It put fun into what collecting elsewhere was just plain hard work.

Just why fossil nodules are abundant in the two major deposits and rare elsewhere is something of a puzzle.  The roof shale closely overlying the coal bed has many fossil impressions but no nodules.  The roof shale is composed of semi-hardened clay silt which disintegrates after long exposure to the weather.  The nodules are embedded in this same type of shale higher up, and they are hardened by iron and sulfur, probably due to bacteria originating in the fossil within.  The clay silt was washed in by floodwater from a neighboring sea.  This arrested plant growth, which was renewed when the water receded.  Successive inroads and recessions of floodwaters resulted in successive coal beds which were given numbers, 1, 2, 3, etc. [by geologists] indicating their periods of deposition.