26
J.S.Smith
3. The bayhead beaches of Banffshire and Kincardineshire form a distinctive
third group of beaches. These are relatively small units, often partially cliffgirt, and their stability is generally assured as a result of their enclosed
location and superimposition on pre-glacial bedrock platforms and ledges.
Offshore Topography and Deposits - The Coastal Sediment Bank
A supply of materials suitable for transport and capable of being moved by waves and
to a lesser extent coastal currents is necessary for any beach form to be created or
maintained. This source may be material removed from another coastal location and
re-deposited or may be derived from an offshore source. Increasing information on the
offshore topography and sub-sea deposits are becoming available through analysis of
Admiralty hydrographic surveys and charts. The distribution of offshore sediments as
far as they can be constructed show clearly that reservoirs of all sizes of material lie
offshore derived from deposits released during deglaciation and glaciation. These
deposits rest on a complex basement with a marked break of slope about 6 km. offshore
marking the junction between the crystalline and sedimentary rocks. The coastal
platform thus identified presumably represents, in part, a lithological boundary and in
part a process-divide, related to marine activities during low glacial sea levels. Notable
bulges of the inshore submarine contours opposite major river mouths represent the
drowned delta fans of the fluvial inputs during low sea levels. In relation to current
fluvial inputs of materials of sand and coarser sediments, it is almost certain that only
the rivers of the Moray Firth coast, notably the Spey, contribute much to the offshore
sediment bank, the lower courses of the others being characterised by highly efficient
sediment traps in the form of estuaries.
The period of shoreline regularisation described earlier in this chapter was
characterised at first by a rising sea level, with subsequent transportation of sea bottom
materials over a wide zone available within wave base. As the forelands and
strandplains were built, and as the shoreline was displaced seawards through land
uplift, so the quantity of material within the reach of waves decreased. Thus the scale
of the coastal forelands reflects large quantities of Pleistocene sediments re-worked by
wave activity over a period of several thousand years. As land-sea relationships became
stabilised at their present position, materials within wave base, even allowing for storm
and surge activity, decreased, as the fresh supplies from cliff erosion and fluvial inputs
are confined to a few specific locations.
The morphological evidence along the coast can be taken as indirect evidence
of the current state of the offshore sediment bank. In interpreting this evidence, care
must be taken to allow for the exceptional storm tide activities likely to result from
temporal correspondence of low barometric pressure and spring tides (the so-called
barometric tide or storm surge), as well as the characteristic seasonal beach profile
cycle, where winter downcombing of the dune edge at the beach back is healed by
summer deflation from the drying upper beach, forming a generally neutral coastal
edge.
J.S.Smith
3. The bayhead beaches of Banffshire and Kincardineshire form a distinctive
third group of beaches. These are relatively small units, often partially cliffgirt, and their stability is generally assured as a result of their enclosed
location and superimposition on pre-glacial bedrock platforms and ledges.
Offshore Topography and Deposits - The Coastal Sediment Bank
A supply of materials suitable for transport and capable of being moved by waves and
to a lesser extent coastal currents is necessary for any beach form to be created or
maintained. This source may be material removed from another coastal location and
re-deposited or may be derived from an offshore source. Increasing information on the
offshore topography and sub-sea deposits are becoming available through analysis of
Admiralty hydrographic surveys and charts. The distribution of offshore sediments as
far as they can be constructed show clearly that reservoirs of all sizes of material lie
offshore derived from deposits released during deglaciation and glaciation. These
deposits rest on a complex basement with a marked break of slope about 6 km. offshore
marking the junction between the crystalline and sedimentary rocks. The coastal
platform thus identified presumably represents, in part, a lithological boundary and in
part a process-divide, related to marine activities during low glacial sea levels. Notable
bulges of the inshore submarine contours opposite major river mouths represent the
drowned delta fans of the fluvial inputs during low sea levels. In relation to current
fluvial inputs of materials of sand and coarser sediments, it is almost certain that only
the rivers of the Moray Firth coast, notably the Spey, contribute much to the offshore
sediment bank, the lower courses of the others being characterised by highly efficient
sediment traps in the form of estuaries.
The period of shoreline regularisation described earlier in this chapter was
characterised at first by a rising sea level, with subsequent transportation of sea bottom
materials over a wide zone available within wave base. As the forelands and
strandplains were built, and as the shoreline was displaced seawards through land
uplift, so the quantity of material within the reach of waves decreased. Thus the scale
of the coastal forelands reflects large quantities of Pleistocene sediments re-worked by
wave activity over a period of several thousand years. As land-sea relationships became
stabilised at their present position, materials within wave base, even allowing for storm
and surge activity, decreased, as the fresh supplies from cliff erosion and fluvial inputs
are confined to a few specific locations.
The morphological evidence along the coast can be taken as indirect evidence
of the current state of the offshore sediment bank. In interpreting this evidence, care
must be taken to allow for the exceptional storm tide activities likely to result from
temporal correspondence of low barometric pressure and spring tides (the so-called
barometric tide or storm surge), as well as the characteristic seasonal beach profile
cycle, where winter downcombing of the dune edge at the beach back is healed by
summer deflation from the drying upper beach, forming a generally neutral coastal
edge.