The bedrock morphology of Start Bay and structures within overlying sediments have been determined by a combined geophysical and geological survey. Analysis of about 50 vibrocore and 200 bottom samples show that three discrete lithologic units, invariably defined by sharp boundaries, can be identified within the Bay. These have been described as barrier, bay and bank deposits. The barrier deposits consist of shingle or beach gravel and occupy a relatively narrow zone extending from the backshore of barrier beaches to about 200 m beyond low water mark. The main constituents are flint, chert and quartzite with a small amount of rhyolite, felsite and granite. In contrast, the bay deposits are composed of medium- to fine-grained sand, with varying concentrations of silt, and whole and broken shell. Although the bay deposits attain a maximum thickness of 28 m, there are several areas in the nearshore zone where they are less than 1 m thick and where bedrock is exposed. The bank deposits are mainly coarse shelly sand and approximately occupy the area of the Skerries Bank which, in fact, consists of a drape of possibly unconsolidated material overlying the bay deposits. A break of slope in the bedrock surface, traced along the entire length of Start Bay, and a step profile in the southern part of the Bay, may be either a legacy of submerged topography produced by vigorous tidal action or, alternatively, the product of subaerial and marine processes. Other significant geological features include buried channels and relict barrier beaches.