For most of 1941, Matthes wrote a chapter for the Physics of the Earth books series from the National Research Council. His text was cited in the June 1949 edition of the Quarterly Journal by the Royal Meteorological Society as "a masterly summary of the characteristics and behavior of glaciers". In the article, Matthes stated that most of the glaciers in the western United States are not remnants from the Pleistocene Era, as had been previously held, but rather are "modern" and formed within the last 4000 years. He identified this phenomenon as the Little Ice Age.
