Sequence stratigraphy is a branch of geology, specifically a branch of stratigraphy, that attempts to discern and understand historic geology through time by subdividing and linking sedimentary deposits into unconformity bounded units on a variety of scales. The essence of the method is mapping of strata based on identification of surfaces which are assumed to represent time lines (e.g. subaerial unconformities, maximum flooding surfaces), thereby placing stratigraphy in chronostratigraphic framework allowing understanding of the evolution of the earth's surface in a particular region through time. Sequence stratigraphy is a useful alternative to a purely lithostratigraphic approach, which emphasizes solely based on the compositional similarity of the lithology of rock units rather than time significance. Unconformities are particularly important in understanding geologic history because they represent erosional surfaces where there is a clear gap in the record. Conversely within a sequence the geologic record should be relatively continuous and complete record that is genetically related. Stratigraphers explain sequence boundaries and stratigraphic units primarily in terms of changes in relative sea level (the combination of global changes in eustatic sea level and regional subsidence caused by tectonic subsidence, Thermal subsidence and load-induced subsidence as the weight of accumulated sediment and water cause isostatic subsidence as a sedimentary basin is filled. The net changes resulting from these vertical forces increases or reduces accommodation space for sediments to accumulate in a sedimentary basin. A secondary influence is the rate of sediment supply to the basin which determines the rate at which that space is filled. Historical development The origin of sequence stratigraphy can be traced back to the work of L.L. Sloss on interregional unconformities of the North American craton. Sloss recognized six craton-wide sequences representing hundreds of millions of years of earth history. In the late 1960s Sloss had several students, notably Peter Vail, Robert Mitchum, and John Sangree, who completed dissertations studying the Pennsylvanian sedimentary rocks of the North American craton and became aware that global changes in sea level could have been responsible for the numerous widespread unconformities in those rocks. During their subsequent careers as research scientists at Exxon's research division Vail, Mitchum and others pioneered the practice of seismic stratigraphy, the stratigraphic interpretation of seismic reflection profiles to understand the layering and packaging of sedimentary rocks in the subsurface using acoustic imaging. The advent of seismic stratigraphy made it possible to identify sequences representing shorter period of time ranging