25.6: *What's in a name?

The Taconian Orogeny is also called “the Taconic Orogeny” by enough geologists that it’s probably worth exploring the different names here. The authors of this text believe that “Taconian” is the better term, and that “Taconic” is misleading. Let us briefly explain why...

The Taconic Mountains are a small, modern mountain range in upstate New York are located east of Albany, on the border with Massachusetts, close to the southwestern corner of Vermont:

Foundational work on understanding Ordovician mountain-building was first completed in these (modern) mountains, and thus the local landmarks provided the name for the orogenic episode. However — and this is the key point — the entire mountain chain from the Ordovician was not limited to the area of the modern Taconic Mountains. Instead, the ancient mountains extended from Newfoundland in eastern Canada all the way down to Alabama.

Not only were the ancient Taconian Mountains much longer as a range than the modern Taconic Mountains, they were taller, too. The highest peak of the Taconic Range today is only about 600 meters tall. In contrast, estimates from metamorphic minerals formed during the Taconian Orogeny suggest that the peaks of the Ordovician-aged Taconian Range must have been much taller. Peak metamorphic pressures of 1.5 GPa imply something on the order of 20 km of overlying crustal material. The Taconian Mountains, in other words, would have been an Alpine-scale range. In the modern Alps, the highest peak is more than 4000 meters tall.

Those ancient mountains are gone now, eroded away over geologic time. We can observe their eroded roots, and we can observe the sediment that resulted from that erosion, but the mountains themselves as topographic features are long gone. That ancient range deserves its own name, and that name should be distinct from the name that’s applied to the modern range. If the modern range is the Taconic Mountains, then the Ordovician mountains need a different name: Taconian.

Let’s summarize with a quick comparison and contrast: The Taconian Mountains were an ancient landscape feature, thousands of kilometers long, with peaks that likely once exceeded 4000 m elevation, and are now completely eroded away. The Taconic mountains are a modern landscape feature, about 20 kilometers long, with maximum elevation of merely 400 m, and the mountains are not yet completely eroded away.

The Taconic Mountains are where the Taconian Orogeny was first described, but we should not confuse the piddly modern mountain range with its mighty Ordovician-aged predecessor. Vastly different in age, height, and extent, they deserve different names.