Geology / U.S.A.

The Silurian in the U.S.A.

It didn't take long after the pioneering work of Murchison in the UK way back in the 1830s for geologists in the United States to start mapping out rocks of Silurian age. In 1850 James Hall from New York compiled the first geological survey of New York state.

In the Silurian time period, the Appalachian Mountains that were uplifted during the Ordovician were being gradually eroded away. The transition from Ordovician to Silurian is not as clearly marked in the US as it is in other areas.

Large reefs of Silurian age are found all over. In Michigan, Wisconsin, Ohio, New York and Alaska thick deposits of Silurian limestone are found. As in the rest of the World, the end of the Ordovician was marked by the inundation of the shelf areas by shallow seas. The impressive Niagara Falls is formed by the dramatic erosion by the Niagara River of the hard dolomitic limestone of the Niagara Escarpment ➚ Silurian limestone. The underlying, softer rocks of Ordovician age are overlain by harder Silurian limestone that was chemically converted to Dolomite ➚ (or dolostone) that forms the hard cap of the Falls.

In Wisconsin the reef limestone has been meticulously studied and shown to contain no less than 191 different species of fossil life. In New York some of the limestone is dolomitic and contains many fossils of Eurypterids probably numerous in shallow lagoons behind the reefs. The Ohio and the Michigan Basin formed vast salt deposits resulting from the inflowing sea water into large evaporational basins. The US was roughly located on the Equator during Silurian times. Alaska has many stromatolite reefs. Stromatolites were the earliest reef building creatures, before the more familiar corals took over as the main constructor organism.

The widely exposed Medina sandstone stretching from Alabama to New York is also of Silurian age.

The end of the Silurian in the US was not marked by a dramatic change in rock deposition, unlike the story in the UK where two plates collided. Indeed the whole Silurian in the US seems to be a time of gentle consolidation in the inexorable expansion of life on Earth.