Complex sentence: He worked at the United States Geological Survey where Cloud was Chief of Paleontology and Stratigraphy from 1951 until 1956, when he was awarded a Guggenheim fellowship for studies in Europe.
Split into simpler sentences: He worked at the United States Geological Survey where Cloud was Chief of Paleontology and Stratigraphy from 1951 until 1956. He left the USGS when he was awarded a Guggenheim fellowship for studies in Europe.