"Detailed measurements of six detachment folds in the northeastern Brooks Range, Alaska, show that the competent Lisburne Limestone, which overlies a regional décollement in the Kayak Shale, varies considerably in structural thickness across folds, with substantial thickening in both the crest and trough panels. Several factors appear to influence this thickening, including variations in mechanical stratigraphy, relative stratigraphic thicknesses of the competent and incompetent units, and structural relief of the underlying basement. My observations indicate that these detachment folds formed in a manner significantly different from that predicted by existing geometric and kinematic models. Thus, I propose a new conceptual model which allows variable amounts of bed shortening and thickness changes in both the competent and incompetent units, thereby opening up an infinite array of possibilities for the development of a fold. My analyses demonstrate that this model provides a better fit to these natural folds than do other models"--Leaf iii.