Episode Three - Final closure of the proto-Atlantic Ocean; Thrusting and Folding
(about 270 million years ago)

Paleogeographic map showing the supercontinent Pangea during the Permian. Note the large mountain range that was built from the collision of North America (Laurasia) and Africa (Gondwana). Diagram courtesy of Dr. Ron Blakey's Global Earth History page, used with permission.
In early Permian time, not long after the marginal mountains had been elevated again in the Late Carboniferous, a more extreme collision took place as the continental mass of Africa collided with the continental mass of Laurentia.(see diagram at right). Compressive stress accompanying this collision was so intense that great portions of the Laurentian crust and overlying sedimentary sequence were thrust westward toward the continent interior. [Figure] Above the thrust planes, the sequence of sedimentary strata many kilometers thick, which had been deposited over hundreds of millions of years (see Episode 1 and Episode 2 above), was warped and folded as the strata were forced toward the west.

A NW - SE hypothetical cross section through central Pennsylvania, illustrating the folding and thrusting of sedimentary layers during the Permian. Modified from R. T. Faill in The Geology of Pennsylvania, 1999. 

That compression did not happen overnight. As ten or more million years passed, strata in the sedimentary sequence underwent a series of deformational stages: first a squeezing up of the sedimentary materials, then the beginning of small internal movements, and finally significant bending and warping of the strata as the thrust planes glided northwestward. (see photos below). In central Pennsylvania, the evidence of this compressive deformation can be seen in the major landscape features of the region, as well as in the bent and folded strata shown in many outcrops.

Folded and thrusted Tuscarora formation near Milroy, Pennsylvania

Kink folds in Trimmers Rock Formation, Watts Exit, Routes 22/322, central Pennsylvania.

continue to Episode 4...


Introduction Plate Tectonics Episode 1 Episode 2 End of Episode 2 Episode 3 Episode 4