Modern archaeology is the discipline of archaeology which contributes to excavations. Johann Joachim Winckelmann was one of the founders of scientific archaeology and first applied the categories of style on a large, systematic basis to the history of art. He was "the prophet and founding hero of modern archaeology". The next major figure in the development of archaeology was Mortimer Wheeler, whose highly disciplined approach to excavation and systematic coverage in the 1920s and 1930s brought the science into the modern era. Wheeler developed the grid system of excavation, which was further improved on by his student Kathleen Kenyon. The two constant themes in their attempts to improve archaeological excavation were first, to maintain strict stratigraphic control while excavating (for this purpose, the baulks between trenches served to retain a record of the strata that had been dug through), and second, to publish a record of the excavation promptly and in a form that would tell the story of the site to the intelligent reader. Bomb damage during the Second World War and subsequent rebuilding gave archaeologists the opportunity to meaningfully examine inhabited cities for the first time. Bombed sites provided windows onto the development of European cities whose pasts had been buried beneath working buildings. Urban archaeology necessitated a new approach as centuries of human occupation had created deep layers of stratigraphy that could often only be seen through the keyholes of individual building plots. In Britain, post-war archaeologists such as W. F. Grimes and Martin Biddle took the initiative in studying this previously unexamined area and developed the archaeological methods now employed in much cultural resource management and rescue archaeology. Archaeology increasingly became a professional activity during the first half of the 20th century. Although the bulk of an excavation's workforce would still consist of volunteers, it would normally be led by a professional. It was now possible to study archaeology as a subject in universities and other schools, and by the end of the 20th century nearly all professional archaeologists, at least in developed countries, were graduates of such programs. New technology Undoubtedly the major technological development in 20th century archaeology was the introduction of radiocarbon dating, based on a theory first developed by American scientist Willard Libby in 1949. Despite its many limitations (compared to later methods it is inaccurate; it can only be used on organic matter; it is reliant on a dataset to calibrate