THE “COAL MEASURES” OF GRAND LEDGE, MICHIGAN, USA

Among the best-exposed outcrops in Michigan’s southern peninsula are Pennsylvanian sandstone ledges exposed along the Grand River in the town of Grand Ledge. Silts, clays, and coals in classic midcontinent ‘cyclothem’ stratigraphy underlie the sandstones. Descriptions of these rocks appear in the first report of the first State Geologist of Michigan, Douglass Houghton, in 1838, and were studied by Alexander Winchell (1860s), Carl Rominger (1870s), W.A. Kelly (1930s) and regularly after by paleontologists and sedimentologists from the State, colleges, and universities. Despite this copious scrutiny, 183 years after Houghton’s report the stratigraphic placement, correlation, and genesis of Michigan’s “Coal Measures” still provide more questions than answers.

The best exposure of a classic cyclothem sequence is in a former clay quarry. After operations ceased, ownership of the land passed to the county and was leased to the City for development as parks. Among the beneficiaries of the repurposed quarry pits are generations of geology students from across the Lower Peninsula. It is hard to overstate the educational value of these exposures. By our estimate, over 100 undergraduate geology students visit the parks each spring and fall, from at least a dozen different educational institutions. It is fair to say that the outcrops at Grand Ledge played and continue to play an important role in the professional development of many practicing geoscientists and geoscience educators in mid-Michigan. Over 100 geologists, from academia, government, and industry, signed a letter supporting preservation of the outcrops after the announcement, in fall of 2020, of plans by the City to expand the town’s water treatment plant, adjacent to the best cyclothem exposure. City officials were unaware of the value of the rocks exposed in their parks and expressed a willingness to work with geologists to preserve the site. This experience underscored the importance of establishing relationships with those who have oversight of these “greatest outcrops” to ensure their preservation for future generations.