Estuarine development was contemporaneous with the onset of glaciation. In the Tay estuary fluvial erosion dominated the downcutting, with minor glacial modification; in the Forth estuary glacial scouring dominated the erosion of rockhead.
Extensive subsurface exploration in connection with road improvement and foundations for bridge building and electricity pylon construction have yielded information on the fill of the Tay and Earn valleys. In addition to geophysical investigations the surveys include boreholes along a 20 km long section from Kinfauns to Longforgan, revealing deposition of lodgement tills followed by Arctic Clays, believed to have been deposited partly from floating ice. Above it are coarse sands and gravels, perhaps outwash deposits. Both were deeply dissected during a fall of sea level associated with a channel cut to − 37 m O.D. at Dundee.
Erosion and marine planation during lowered sea levels produced laterally extensive buried gravels.
A rise of sea level led to burial of the eroded surface by early estuarine sediments which become finer upwards and terminate in peats, dated at 8500 b.p. Further erosion was followed by a second phase of estuarine sedimentation. This produced the carse deposuts whose terminal peats accumulated at 5500 b.p.
The modern estuarial deposits partly bury those of the two preceding estuaries, which exert a major control on the migrations and depth of the modern channels.