Stratigraphy has long been a descriptive science, rather than experimental, largely because of the great number and variety of variables involved. However, stratigraphic models can be constructed, based on the generalization that the shape of a body of sedimentary rocks varies as a function of the quantity of material supplied to the depositional site, the rate of dispersal, and the nature of the material supplied. Within the framework of a generalized continental model, specific environmental models can be distinguished. Subvariants of cratonic source models include: 1) equal subsidence, 2) linear increase in subsidence, and 3) non-linear subsidence. Subvariants of external source models include: 1) clastic wedge models, and 2) turbidite models.