Geomorphology of a Sand Ridge

In the past, sand ridges and the associated marine sand waves have been studied extensively from a morphological point of view, but several important historical and dynamical problems associated with these features have been neglected. This paper outlines the present dynamical setting of a sand ridge (a shoal named Middle Ground) in Vineyard Sound, Massachusetts, then discusses its origin. Sand waves cover the ridge. Those with amplitudes of 2 m. are found to be completely rebuilt in half a tidal cycle, and those with amplitudes of 6 m. are found to be significantly rebuilt over half a tidal cycle. Net movements of the sand waves are consistent with the counterclockwise net circulation of water around the ridge. The velocity profile is logarithmic only over the sand-wave crests where the average maximum shear velocity is 4.5 cm/sec. The results of a grab sample study, when combined with dynamical considerations, lead to the conclusion that Middle Ground is a body of sand, whereas its southwest extension (Lucas Shoal) is composed of glacial drift and veneered with sand. By the use of these and other data, the geomorphological history of Middle Ground is traced from the deposition of a small Wisconsin recessional moraine, through a period of sea level rise during which wave erosion and longshore drift created an elongate submarine sand ridge, to a period of submergence during which waves modified the relic morainal feature and tidal currents reshaped both the submerged sand ridge and the relic moraine. Theory and field measurements indicate that the present morphology of the shoal is in dynamic equilibrium with the tidal current system, and that large-scale morphological changes occur within a period of only a few years.