THE VALUE OF ‘STAGE’ IN COASTAL GEOMORPHOLOGICAL MAPS (Invited Presentation)

The tripartite foundation for geomorphological mapping incorporates the components of causative process, form of the surface feature, and stage in development. Because most of the surface features on coastal barrier systems are ridge and swale forms related to dune forming processes associated with variations in sediment budget, this landscape presents considerable similarity in form and process. As a result, the discriminating variable separating the myriad of dune forms is the relative stage of development. The application of this discriminator is accomplished through a characterization of the spatial distribution of specific dunal features utilizing their relative positions and orientations as indicators of sequence in dune development and in barrier evolution. Additionally, secondary processes acting over time create alterations of form, assisting in the assignment of ‘stage’ in the dunal topography. ‘Stage’, therefore, leads to the creation of categories of geomorphological features for the geomorphological map.

Nearly all of the dunal features on the barrier islands originated as foredunes in active sediment exchange with an adjacent beach. Thus, the most basic separation distinguishes dune features that are actively developing versus those that are currently occupying some position other than adjacent to an active beach, creating categories of active foredunes and abandoned foredunes. Further segregations in sequence are made within the abandoned category, distinguishing between abandoned ridges associated with the modern shoreline and those associated with ancestral barrier positions. The “modern” abandoned ridges are former foredunes that are related to the active foredunes and they constitute a distinct stage in the barrier island evolution, albeit more recent than the ancestral forms. The “ancestral” foredunes are associated with a different island configuration and they constitute yet another stage, spatially and topographically distinct from the active dune/beach system and the “modern” abandoned foredunes.

Recent geomorphological maps for Fire Island National Seashore, Gateway National Recreation Area, and Assateague Island National Seashore exemplify ‘stage’ in coastal dune development as a central element in assessing the evolution of barrier systems.