By the 1960s, however, the quantitative revolution led to strong criticism of regional geography. Due to a perceived lack of scientific rigor in an overly descriptive nature of the discipline, and a continued separation of geography from its two subfields of physical and human geography and from geology, geographers in the mid-20th century began to apply statistical and mathematical models in order to solve spatial problems. Much of the development during the quantitative revolution is now apparent in the use of geographic information systems; the use of statistics, spatial modeling, and positivist approaches are still important to many branches of human geography. Well-known geographers from this period are Fred K. Schaefer, Waldo Tobler, William Garrison, Peter Haggett, Richard J. Chorley, William Bunge, and Torsten Hägerstrand.