The Rochester shale comprises the upper 85’-90’ of the Middle Silurian Clinton Group in upstate New York and southern Ontario. The upper two-thirds of the unit contain numerous grey limestone beds, which can be sufficiently resistant to form outcrops, but which are poor in fossil content. The lower 20’ of the Rochester shale is notoriously susceptible to erosion such that outcrop exposure is rare. Unfortunately, it is this rapidly weathering unit at the base that contains the abundant fossil assemblage (specifically trilobites) for which the formation is famous. The best visible exposure of the full Rochester shale section is in the gorge walls of High Falls on the Genesee River. However, this section is agonizingly unapproachable for fossil collection, particularly so the lower 10-20’ of section best known for fossil diversity and abundance.
Over 200 invertebrate fossil species have been identified from the Rochester shale including corals, brachiopods, bivalves, gastropods, cephalopods and crinoids. But, it is the spectacularly preserved trilobites that have attracted collectors to the Rochester shale since James Hall first detailed their occurrence in the 1840’s. The diverse fauna combined with the fine grained shale beds and thin limestone units indicate that the Rochester shale was deposited in warm, well oxygenated marine waters of intermediate depth. But where does one find outcrop to search for fossils?
If Time is Relative, Geologic Time is Exponentially Relative
The geologic time scale is a difficult concept for humans to appreciate. We live less than 100 years, the Vikings came to America 1000 years ago, our current calendar just passed the 2000 year mark, and Stonehenge dates almost 5000 years old. That, we say, was a long time ago. But these are mere seconds on a geologic clock. Even the final Ice Age advance that generated upstate New York’s topography and fertile soil ended a mere 12,500 years ago.
Now think about this. Dinosaurs roamed and dominated much of the planet for the entire Mesozoic Era. For over 160 million years (MY) they lived, and died until going extinct about 65 million years ago. By comparison, humans have been inhabitants for just under one million years. And all but the last 6000 years or so of that is referred to as the Stone Age, the period before metal was worked and likely before crops were cultivated.
Geology in Action, Wolf Creek Dam, Lake Cumberland, Kentucky
The construction of Wolf Creek Dam in central Kentucky began in 1941, but work was interrupted by WW2 and the dam was not completed until 1951. Potential problems with the integrity of the underlying Ordovician Leipers limestone were appreciated during construction and extensive cement was placed in a number of cave features that were known to exist before the earthen dam was built above. However, that early work proved to be insufficient in preventing seepage from Lake Cumberland through the underlying karst.
1947 photograph highlighting cavernous regions in the dam’s base that were filled with cement.
