The Bronson Hill Arc is a bimodal volcanic arc and associated sediments that formed over a west (?) dipping subduction zone during the

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

period (c. 475 - 450 million years ago (Ma)) as part of the

Taconic Orogeny The Taconic orogeny was a mountain building period that ended 440 million years ago and affected most of modern-day New England. A great mountain chain formed from eastern Canada down through what is now the Piedmont of the East coast of the Unit ...

. These rocks are presently well exposed along the

Connecticut River Valley The Connecticut River is the longest river in the New England region of the United States, flowing roughly southward for through four states. It rises 300 yards (270 m) south of the U.S. border with Quebec, Canada, and discharges at Long Island ...

Vermont Vermont () is a state in the northeast New England region of the United States. Vermont is bordered by the states of Massachusetts to the south, New Hampshire to the east, and New York to the west, and the Canadian province of Quebec to ...

and

New Hampshire New Hampshire is a state in the New England region of the northeastern United States. It is bordered by Massachusetts to the south, Vermont to the west, Maine and the Gulf of Maine to the east, and the Canadian province of Quebec to the nor ...

. The arc is evidenced by plutonism and

extrusive Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. In contra ...

volcanicsm, including the Ammonoosuc Volcanics (c. 461 Ma from U/Pb

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of t ...

dates) and the overlying Partridge Formation (c. 457 Ma from

graptolite Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian ( Miaolingian, Wuliuan) through t ...

s in the formation). It is related to the slightly older Shelburne Falls arc that sits to the west. These rocks were

metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

and deformed during the Acadian Orogeny and the

Alleghenian Orogeny The Alleghanian orogeny or Appalachian orogeny is one of the geological mountain-forming events that formed the Appalachian Mountains and Allegheny Mountains. The term and spelling Alleghany orogeny was originally proposed by H.P. Woodward in 195 ...

References

*Robert H. Moench and John N. Aleinikoff, "Stratigraphy, Geochronology, and Accretionary Terrane Settings of Two Bronson Hill Arc Sequences, Northern New England", ''Physics and Chemistry of the Earth, Parts A/B/C'', Volume 27, Issues 1–3, January 2002, pages 47–95. Geology of New Hampshire Geology of Vermont Terranes {{US-geology-stub