Mountain Building Part III: the Acadian Mountains

Figure 1.13: North America and Baltica collided finally in the mid-Devonian, crumpling the crust to form the Acadian Mountains. Sediments eroded from the highlands formed the Catskill Delta. Figure by J. Houghton.

When Baltica (proto-Europe) finally collided with North America around 380 million years ago in the middle Devonian, the exotic terranes making up New England were in between the colliding continents. The terranes (and the eastern margin of North America) were squeezed, folded, metamorphosed and intruded by magma. This collision with Baltica and North America formed yet another tall mountain chain, the Acadian Mountains, along the eastern margin of North America. The Acadian Mountains were siimilar to the Taconic and Grenville Mountains of the past which had since eroded (Figure 1.13). Just as in the Taconic mountain-building period, compression from the Acadian continental collision warped the crust downward, reinforcing the inland ocean. The Queenston Delta was buried by new sediments eroding from the western side of the Acadian Mountains. These sediments, known as the Catskill Delta, created a new wedge of sediments stretching into a shallow inland sea.

Figure 1.14: Devonian: 390 million years ago.

During this time, North America gradually began to move closer to its present geography and assume the north-south alignment we see today. At the time of the Acadian mountain building and subsequent erosion during the Devonian, the Northeast was at the Equator and experiencing the associated tropical climate (Figure 1.15). Baltica (proto-Europe) and North America were united as one larger landmass. Africa, South America, India, Australia, Antarctica and Florida were all combined as one continent (Gondwana) in the southern hemisphere. The continents were gradually merging to become one.

Figure 1.15: Acadian Mountain Building:

  • Baltica collides with North America.
  • Acadian Mountains form (Northern Appalachian Mountains.
  • Similar to Taconic Mountain building.
  • Inland sea forms west of Acadian Mountains.
  • Acadian Mountains erode.
  • Catskill Delta deposited west of Acadian Mountains.

Between mountain-building events: deposition in the inland ocean

The Northeast was not continuously experiencing dynamic mountain-building events. There were quieter times as well between the rise of great mountains and crushing crusts of colliding plates. The quiet times were marked by erosion of the highlands and very little plate movement and compression within the Northeast region. The building of the Taconic Mountains was over by the late Ordovician. Throughout the following Silurian period, the Northeast experienced a quiet time in which erosion from the Taconic highlands and deposition in the inland sea were the main events. Huge thicknesses of sedimentary rocks accumulated in and on the margins of the inland sea during part of the Silurian. The inland ocean, which spread across much of New York, Pennsylvania and western Maryland, was similar to the modern Persian Gulf, becoming very salty because of the shallow water, high rates of evaporation and poor circulation.