Thomas Benton Brooks

2023 ILSG Pioneer

Shortly after the U.S. Civil War Major Thomas Benton Brooks moved to the Marquette Iron Range. There over the course of less than a decade, he became the premier geologist, prospector, mining and civil engineer, and mining company executive of the region.  During these formative years of the iron ore industry, when the Lake Superior region was providing about one-quarter of the iron ore used in the U.S.,  he was employed by the Iron Cliffs Company, the predecessor of the Cleveland-Cliffs Company, the Michigan and Wisconsin Geological Surveys, and served as a consultant to iron ore exploration and mining companies of the region. His contributions had a significant role in mapping the Precambrian geology and iron ranges of Michigan and Wisconsin and a lasting impact on the iron ore industry of the region. As stated by Prof. C.R. Van Hise, Brooks’ successor as the premier geologist of the Lake Superior region : “Notwithstanding  the immense advantage which it has been to have Brooks’ work as a foundation, it has taken many years of labor fairly to complete the structural story to which Brooks contributed important chapters. Only those who have labored in the Lake Superior region and who understand its peculiar difficulties can give Brooks credit for the remarkable work he did. His geological work is my ideal of what should be done in a new region of complex geology.”
Thomas B. Brooks was born on June 15, 1836 in Monroe, NY, near the New Jersey border, and died nearby on November 22, 1900. In 1852 at the age of 16, he joined a surveying crew of the Erie Railroad and rapidly advanced from woodsman to instrument man. In 1853 he was employed with the New York Topographic and Geological Survey and then entered the Engineering Department of Union College of Schenectady, NY in 1856, graduating in 1858 in civil engineering. He remained at Union College as an instructor for a year and then took part in topographical surveys in New York, New Jersey, Pennsylvania and the U.S. Gulf Coast. In 1860 he attended a series of lectures on geology given by Prof. J.P. Lesley former state geologist of Pennsylvania and Professor of Geology at the University of Pennsylvania. This was his only formal education in geology.  He volunteered for the Union Army in 1861 and organized an engineering company that had a distinguished record during numerous Civil War campaigns. He retired from the Union Army in 1864 as a brevet colonel after being wounded in the battle of Denly’s Bluff, but referred to himself after the war as Major Brooks.

            In 1865 after leaving the Union Army he accepted a position with the Geological Survey of New Jersey where he conducted magnetic surveys with a dip needle to locate iron ores and was put in charge of mines and furnaces. Shortly thereafter, he was induced to take charge of the mines of the Iron Cliffs Company in the Marquette Iron Range as vice-president and general manager. He moved to Negaunee, Michigan, where his practical knowledge of geology and engineering, leadership skills, originality, keen powers of observation and deduction, and intense work ethic served him, the company, and the Lake Superior region well. This is where his extensive geological studies began and where he developed the instruments and methodology to exploit the iron ores of the Lake Superior region. He brought the dip needle to the Lake Superior region and was among or possibly was the very first, to use it in iron ore exploration and geologic mapping in the region. He also pioneered the dial (Sun) compass, which he modified for geologic use from the surveying solar compass developed by W.A. Burt.

In 1869 he resigned from the Iron Cliffs Company and was given the responsibility of mapping and reporting on the Marquette Iron Range and was placed in charge of the Economic State Geological Survey of the district by the Michigan Geological Survey, essentially becoming the State Geologist of the Northern Peninsula. He received no salary for this position, but he was allowed to receive private funds from numerous iron ore companies and mines. Unfortunately, his intense work schedule took a toll on his health that caused him to leave Marquette with his family in the winter of 1872-73 for London, England and eventually Dresden, Germany, where he hoped to regain his health, but failed to do so. During this period he prepared reports on his iron range geologic studies for publication by the Michigan and Wisconsin Geological Surveys (Brooks, 1873 and 1880), articles on the geology of the region and magnetic surveying instruments and their use published in various journals including the American Journal of Science and Arts (Brooks and Pumpelly, 1872; Brooks, 1875), and co-authored the book “Iron Ores of Missouri and Michigan” (Pumpelly, Brooks, and Schmidt, 1876).

            During his years involved with the geology and ores of the Lake Superior region Major Brooks made numerous advances in the geological knowledge of the region that have served as a foundation for future studies and developed methods and instruments that proved useful for exploiting the ores of the region for many years. The following are a list of his major lasting accomplishments:

  • He with the assistance of R. Pumpelly and R.D. Irving developed the dial (Sun) compass for geologic studies based on the principal of Burt’s surveying solar compass which together with the dip needle that he brought from the Geological Survey of New Jersey were used in the Lake Superior region for nearly a century to locate and outline iron-rich rocks and ores. His publications on these instruments led to their extensive worldwide use.
  • He established procedures for conducting magnetic surveys for geological purposes in the Lake Superior region and methods of interpreting the observations of the surveys based on empirical studies.
  • He was the first to describe  the magnetic characteristics of the minerals and rocks of the Lake Superior region.
  • He (Brooks, 1872a) recognized that magnetic anomalies observed in the area of non-magnetic Paleozoic (then Silurian) sedimentary rocks of the eastern part of the Northern Peninsula of Michigan were likely derived from the basement Precambrian rocks that crop out to the west. Accordingly, these anomalies could be used to trace the basement rocks and their structure beneath the sedimentary rocks. Furthermore he realized that anomaly characteristics could be used to determine the depth to magnetic sources and thus, the thickness of the sedimentary rocks. In a similar manner he understood that perhaps the depth of Lake Superior could be determined from analysis of the lake magnetic anomalies.
  • He founded the first assay facility for iron ores in the Lake Superior region in the city of Marquette which facilitated iron ore mining in the region.
  • He conducted one of the first geological surveys of the Marquette, Menominee, Crystal Falls, and Gogebic Iron Ranges. He was the first to understand that the Marquette Iron Range occurs within a 75-km long syncline extending to the west from near Marquette, Michigan (Allen and Martin, 1922).
  • He recognized the stratigraphic position of the copper-bearing rocks of the Northern Peninsula of Michigan and suggested the name Keweenawian (note his spelling) for the age of these rocks in American Journal of Science and Arts articles of 1872 and 1875. Subsequently, the term Keweenawan has been used for these rocks.
  • He had an important role in developing safe, efficient methods of mining iron ores of the Lake Superior region (Brooks, 1972b).
  • He was intensely interested in the education of his children and supported the studies of his son, Alfred Hulse Brooks, a famed geologist of the U.S. Geological Survey, Alaska Branch, who is honored by naming of the Brooks Range of Alaska after him.

These are all significant contributions that have had a profound role in understanding of the geology of the Lake Superior region and the exploitation of its ores. They have largely gone unrecognized for the past century and a half, but they  clearly distinguish Major Thomas Benton Brooks as a Pioneer of Lake Superior geology.


Allen, R.C., and Martin, H.M., 1922. A brief history of the Geological and Botanical Survey of Michigan. Michigan History Magazine, Volume VI, No. 44, 675-750.
Brooks, T.B., 1872a. On the use of the magnetic needle in mineral explorations on Lake Superior. Van Nostrand’s Eclectic Engineering Magazine (1869-1879), August 1, 1872; Volume 7, No. 44, American Periodicals, 161 p.
Brooks, T.B., 1872b. An analysis of the cost and description of the methods of mining employed in the Marquette Iron Region, Lake Superior, Michigan. Transactions of the American Society of Civil Engineers, Volume XXXIV, 18 p.
Brooks, T.B., and Pumpelly, R., 1872. On the age of the copper-bearing rocks of Lake Superior. American Journal of Science and Arts, Third Series, Volume III, No. XVIII, , 428-432.
Brooks, T.B., 1873. Geology of Marquette Iron Range, Geology of the Menominee Iron Range, and Geology of the Gogebic and Montreal Iron Ranges. Michigan Geological Survey, Volume 1, Chapters IV, V, VI, VII, and VIII, Part 1, Iron-Bearing Rocks, 117-243.
Brooks, T.B., 1875. On the youngest Huronian rocks south of Lake Superior and the age of the copper-bearing series. American Journal of Science and Arts, Third Series, Volume III, No. XI, 206-211.
Brooks, T.B., 1880. Geology of the Menominee Region.  In Chamberlin, T.C. (ed.), Geology of Wisconsin, Volume 3, Part 7, Chapters 1, 2, and 3, 430-552.

Lawton, C.A., 1900. The Late Major Thomas Benton Brooks: Biographical Sketch of a Man Whose Name is Intimately Associated with the Early Development of Michigan’s Iron Mines. The Daily Mining Journal, November 29, 1900.
Pumpelly, Raphael, Brooks, T.B., and Schmidt, A., 1876. Iron Ores of Missouri and Michigan. G.P. Putnam’s Sons, New York, 624 p.

Willis, B., 1901. Thomas Benton Brooks. Proceedings of the American Association for the Advancement of Science, Science, New Series, Volume 13, No. 325, 460-462.


Submitted by: William J. Hinze, Purdue University