The glacial theory was first proposed by Swiss naturalist and teacher Louis Agassiz in 1837. His idea that the earth had experienced very cold climates and an expansion of glacial ice explained erratic boulders and striated bedrock across Europe. He came to the U.S. to teach at Harvard and found further evidence for his theories. Earlier conclusions about the distribution of boulders attributed the phenomenon to the great flood. A detailed reading of glaciation in Minnesota was started by N.H. Winchell, head of the newly created Minnesota Geological Survey, in 1872. He was assisted by New England glacial geologist Warren Upham. Within 10 years they had completed a great deal of fieldwork mapping moraines and presenting an accurate view of the ice border in North America.