J.A. Udden (1859-1932) was a pioneering American geologist who did a lot of important work in micropaleontology (the study of fossilized microorganisms) and in stratigraphy and sedimentology. His studies of wind-blown sediment were particularly important; he was one of the first geologists to demonstrate that the Pleistocene loess in the Upper Mississippi Valley was wind-deposited rather than water-deposited. He is perhaps best known for his development of a scale for measuring grain size in sedimentary rocks; today, this scale is known as the Udden-Wentworth scale.
He was a geology professor at Augustana College in Rock Island, Illinois from 1888 to 1911, and he joined the University of Texas Mineral Survey in 1904. He later became the director of the Bureau of Economic Geology, another part of The University of Texas at Austin.
After he left academia, he did important work for the petroleum industry. He helped establish sedimentary petrography as an important field; he was one of the first to use rock taken from well cuttings to determine the economic relevance of the rocks being drilled into.
He also did a lot of work in the Glass Mountains of West Texas, and he discovered a new species of ammonite (Uddenites) that has become an important index fossil.
Because of the importance of his geology work, he was knighted by the King of Sweden.