Henry Darwin Rogers was one of the first professional geologists in the United States. He directed two of the earliest state geological surveys—New Jersey and Pennsylvania—in the mid-1830s.  His major interest was Pennsylvania, with its Appalachian Mountains, which Rogers saw as great folds of sedimentary rock. He belived that an interpretation of these folds would lead to an understanding of the dynamic processes that had shaped the earth. From Rogers' efforts to explain these Pennsylvania folds came the first uniquely American theory of mountain elevation, a theory that Rogers personally considered his most significant achievement.