In many respects, the mountain building of the Acadian Orogeny never really came to an end. Following the collision of Avalonia with the eastern margin of Laurasia with the subsequent creation of the Acadian Highlands, Gondwana and South America, now joined together into a single continent, were approaching from the east and south respectively. Even before the continent-continent collision that would occur at the close of Permian time, the highlands of Acadia along the eastern margin of Laurasia and Llanoria along the southern margin were being maintained as uplands in response to the compressional forces generated by the converging continents. In addition, the close of the Mississippian saw a general retreat of the sea from most of the Laurasian craton in response to the increase in Gondwana glaciation. As a result, the Pennsylvanian Period opens with highlands extending along the eastern margin of Laurasia from Newfoundland to Alabama and continuing across the region of the Gulf as far west as New Mexico (Figure 1). Apparently most of eastern Canada was emergent with a shallow sea covering the western portion of the craton as far west as the Rocky Mountain states. However, from Texas and Oklahoma to the eastern margin of the continent, elastic sediments from the bordering highlands held back the sea and filled both the Appalachian and Ouachita basins as rapidly as they sank. The result was the creation of a vast lowland/coastal plain occupying most of the eastern portion of the continent covered with lush forests and great swamps with a shallow sea to the west (Figure 2). East of the hundredth meridian, fluctuations in sea level in response to Gondwana glaciation resulted in the sea moving back and forth across much of the eastern interior with at least a few marine transgressions driving as far east as the Appalachian Basin.
Renton, John J. and Repine, Thomas, "The Pennsylvanian Period" (2016). Readings and Notes. 16.