During deglaciation, large supplies of unconsolidated sediments were contributed to St Andrews Bay by glacial and fluvioglacial processes. These mobile supplies, combined with changing relative sea level, have given rise to considerable modification of the planform of the embayment over the last 15,000 years. The early history is poorly dated and
development has been neai'shore erosion and sedimentation resulting in bathymetric changes which, in part, can be evaluated from the sequence of subsurface sediments beneath the floor of the bay.
Most changes in the area have taken place in the nearshore zone where there is evidence for shoreline regression and progradation. These changes are predominantly recorded at the head of the bay between Carnoustie and St Andrews whereas the rocky cliffed coastlines show limited evidence of change from the time of the ice withdrawal.
Despite the data presented in the shoreline diagram (Fig. 2.5) the evidence for the former shoreline of the bay is fragmentary, poorly preserved and poorly dated prior to the Holocene. Only one shoreline, the Main Perth Shoreline', can be traced around the majority of the embayment (Fig. 2.5). However, although it is defined in the outer part of the bay, the inland extent is subject to debate with some workers (Cullingford, 1972) arguing for a westward penetration of the sea to Perth, whereas others (Armstrong et al., 1985) have argued for marine conditions extending to Crieff some 15 km west of Perth. Uncertainty about the extent of the Tay Estuary leads to a poorly-defined understanding of the sediment inputs to the coastal zone and possible sedimentation processes at that time; i.e. to what extent sediment was glacially or fluvially derived.
Conditions in the bay during this time have been investigated by sub-bottom profiling and boreholes by the British Geological Survey. This work shows the presence of an extensive glaciomarine unit, the 'St Abbs Beds', which covers the area and is believed to date from this time. The faunas in the beds indicate deposition in an environment of arctic character (Hall and Jarvis, 1989). The sediments aie at least 20 m thick over much of the Bay and are found onshore particularly in the Tay Estuary (Browne and Jai'vis, 1983). The presence of this material is believed to be important in the subsequent evolution of the bay since it would have provided a major source of unconsolidated sediments which could be remobilised by wave and tidal activity during the period of low sea level at the end of the Lateglacial.
The shape of St Andrews Bay at the Lateglacial minimum sea level is uncertain. A channel entrenched through the St Abbs Beds sediments to a depth of 40 m below OD in the Tay Estuary is graded to a low sea level in St Andrews Bay (Buller and McManus, 1972). The subsurface erosion surface identified in the bay also relates to this time and suggests that much of the bay was then dry land .
A clearer picture of the changing nature of the bay emerges with the start of the Holocene and the main postglacial transgression which took place from about 10,000-6,000 BP. At the maximum extent of the transgression, the shoreline stood some 7-9 m above its present level at St Andrews and the head of the bay was defined by a cliffline extending from St Andrews (GR. 510158) to Guardbridge (GR. 455195) and from St Michaels (GR. 445230) to Tayport ( GR. 465287). The higher sea level also entered shallow re-entrants in the cliffline between St Andrews and Fife Ness and on the northern shore of the bay. In the estuaries and coastal zone of the postglacial sea, sequences of silt, sand and clay , the "carse clays" were laid down containing a fauna of Boreal aspect and occasional peat beds associated with periods of emergence (Sissons, 1967).
These postglacial deposits were cut into during the period of falling sea level after the transgression and the present planform of the embayment started to emerge from this time. Most significant has been the abandonment of the postglacial cliff line and the eastwards progradation of the shoreline at the head of the bay. Clearly, the present Eden estuaiy is a feature of the postglacial regression in the area, since at the maximum sea level the estuary would have only existed landwards of Guardbridge. Also dating from this period has been the growth of the Tentsmuir platfoim which is composed of windblown sand overlying intertidal sands and occasional peat beds. This process of accretion continues at the present day. North of the Tay the growth of Buddon Ness (Fig. 2.1) and the dune complexes between Carnoustie and Arbroath may be seen as a direct response to falling sea levels during the regression and the period of stability since about 3,000 years BP.