Complex sentence: He left both of the aforementioned posts in 1957, when he became the Dean of Agriculture at his alma mater, Purdue University; in 1968 he was promoted to the positions of Dean of Education and vice president of the university's research foundation.
Split into simpler sentences: He left both of the aforementioned posts in 1957, when he became the Dean of Agriculture at his alma mater, Purdue University. In 1968 he was promoted to the positions of Dean of Education and vice president of the university's research foundation.