the collision creating the acadian orogeny resulted in the closing of the southern iapetus ocean and the formation of a high mountain belt. after the acadian collision took place, gondwana began to retreat from laurentia with the newly accreted avalonian terranes left behind. as gondwana moved away, a new ocean opened up, the rheic ocean, during the middle to late devonian, and subsequently its closure would result in the formation of the alleghanian orogeny.