The sediments of the floor of the Tay Estuary are dominated by assemblages of quartzo-feldspathic sands, although pebbles and silts are locally important. The sands are principally of inorganic materials, but they include shell fragments which account for up to 25% of the material, depending upon locality.
Detailed analyses of the shell components of the 0.3–1.2 mm size fraction (400 particles from each site) in the area between Balmerino and Monifieth Bay reveal the presence of much molluscan debris (identified as mature and immature bivalves and gastropods), with significant proportions of bryozoan, barnacle or echinoderm skeletal fragments according to locality. Ostracoda and foraminifera are less common. The shelly material occurs mostly within zones inhabited by the same living organisms.
The characteristic blue-coloured Mytilus edulis shell fragments are also recognised up to 6 km landwards of the westernmost known colonies, along the railway bridge.
Shell matter is dissimilar in shape and surface texture to the clastic grains and large shell fragments are frequently associated with sand sized inorganic sediments, which have been shown in flume experiments to respond in a similar manner to hydraulic controls.
The Mytilus is therefore suitable for use as a natural tracer. It indicates up-estuary net residual motion of sediments in the middle and upper reaches. This pattern confirms evidence from heavy mineral assemblages that North Sea derived sands enter the estuary and migrate westwards to accumulate on the upper estuarine tidal flats.