March 29. Taconic Orogeny

The ends of the periods of geologic time tend to be marked by major events – extinctions, glaciations, and mountain-building, at least in Britain and the United States, where most of the big packages of geologic time were first described. The Ordovician is no exception.

The Taconic Orogeny is the mountain-building event – that’s what orogeny means – that began toward the end of the Ordovician Period in eastern North America. This was the first significant pulse in the construction of the Appalachian Mountain system, a process that took many millions of years.


The Taconic Mountains today lie along the eastern border of New York, and in western Vermont, Massachusetts, and Connecticut, but the Taconic Orogeny really extends south into Georgia. The collision that caused the mountain uplift involved a complex mess of masses, including some small continental blocks, an extensive island arc system, maybe some slices of oceanic crust, and the sediments that were among all those pieces – but most simplistically, it was probably a volcanic island arc. I’ve used the analogy of the modern western Pacific, and I think that’s a reasonable view. Something like Kamchatka-Japan-Taiwan-Philippines or Indonesia, slamming into a rigid North American continental block, over a period of several million years. And not at the same time, and not in the same way everywhere along its length.

There’s evidence for the beginning of the convergence even in Cambrian and early Ordovician times, in the types of sedimentary rocks we find, but the effects really started to be felt in late Ordovician time. The culmination of the Taconic Orogeny, the uplift of a significant mountain range along the east side of North America, was near the end of the Ordovician, something like 455 to 445 million years ago. The evidence is in the modern Taconic Mountains and in the subsurface elsewhere, where we see the deeply eroded roots of that Ordovician mountain range.

The Queenston Delta, which we discussed on March 21, was one of the most significant consequences of the Taconic Orogeny, at least in North America.

Ebenezer Emmons, a geologist in New York in the 1840s, defined the Taconic Orogeny. He had already described the Potsdam Sandstone and other specific elements of early Paleozoic geology. Emmons ascribed his Taconic System of rocks to the Cambrian – a view vehemently opposed by James Hall, the State Geologist of New York. Hall felt that the rocks belonged to the Silurian. This bitter controversy – called the greatest in American geology – was analogous to the debate going on in Britain, between supporters of Sedgwick and Murchison over the same part of the geologic section. In New York, Emmons was banned from practicing geology; he sued Hall for slander and libel, but lost, and moved to North Carolina. In 1888, 25 years after Emmons’ death, he was ultimately vindicated, as the new Ordovician Period was defined, putting Emmons’ rocks into the Cambrian as he had originally claimed. But it wasn’t until 1903 that the U.S. Geological Survey formally accepted the Ordovician Period.

The present-day Taconic Mountains are mountainous not because of the Taconic Orogeny. The Ordovician mountains were long since eroded to low hills, or less. Later events have pushed the rocks there back up. Building the Appalachians was a long process which we will touch on for many months – many million years – to come.

—Richard I. Gibson

Callan Bentley’s outstanding animation

Another useful link from Paleontological Research Association


Emmons image – public domain
Cross sections from USGS – public domain