James Hutton, who did most of his work in the late 18th century, may have been the first famous geologist. Hutton was the first to formally propose that the Earth continuously renews its rocky crust and that the planet is vastly older than classical theories anticipated. Another geologist, Charles Lyell, largely built on Hutton's work during the 1830s.
Paleontology is a specialized discipline within geology. Mary Anning, who lived from 1799 to 1847, may have been the first great paleontologist. From her youth, Mary showed a remarkable ability to find and extract fossils in the rich beds of southern England. Another paleontologist, William Smith, worked out a method for dating rock strata based on the presence of indicator fossils. This allowed the relative dating of layers of rock for the first time. Smith also produced some of the earliest maps on which the location and extent of rock formations was plotted.
Friedrich Mohs was a German minerologist who did the foundational work on the relative hardness of minerals, which came to be known as the Mohs scale. Another great geologist, and one who wasn't vindicated until after his death, was Alfred Wegener. His ideas about continental drift, later known as plate tectonics, became foundational to modern geology.