A Method to Determine Shingle Supply to the Coast

Abstract

Relict coastal landforms in Montrose Bay are dominated by shingle particles, which are thought to be derived mainly from inland sources. Standard sediment size and shape parameters and lithology and surface textures of sedimentary particles were used to test this hypothesis. Samples from potential sources of supply to the coast were compared with the properties of relict coastal shingle using bivariate scattergrams. The inland shingle was readily distinguishable from the littoral source types. The relict coastal shingle closely resembled the former. A number of shingle supply phases are distinguished beginning with the Late Devensian deglaciation. Shingle was carried by proglacial streams and later by rivers and deposited in the coastal and offshore zones. It was reworked by marine and river action, particularly during the high sea-level phase of the Flandrian Transgression, and deposited along the coast in the form of shingle bars and river terraces. The use of sediment parameters, along with morphological and stratigraphical evidence, could be applied to other areas to ascertain sediment sources and patterns of supply. THE literature discussing the origin of beach shingle in southern Britain (C. Kidson, 1961, 1966) suggests a supply of material from terrestrial sources with subsequent reworking by marine processes. Research in the Montrose area has attempted to test this hypothesis using standard sedimentary particle parameters. Certain relationships have been shown to exist between the fluid media of transport and deposition and the physical characteristics of the sediment in these media (J. R. L. Allen, I970). Therefore, by measuring the size and shape characteristics of individual particles it should be possible to deduce the sources of shingle deposited along a coast. Furthermore by analysing the morphology, stratigraphy and sedimentary properties of shinglebased landforms a chronology of shingle supply and distribution can be elaborated. It was also hoped to assess the applicability of standard size and shape parameters, derived during studies in the large unigenetic environments of the southern United States, to the differentiation of sedimentary environments in a coastal area of upland Britain. However, this is hindered by the diverse genesis of sedimentary particles in the area since the Late Devensian deglaciation. Here, few particles have been in any one environment sufficiently long for their physical characteristics to be moulded to the equilibrium state which reflects that particular environment. To test both the hypothesis and the method, the sedimentary coast between Montrose and St Cyrus in eastern Scotland was selected (R. S. Crofts, 1971 a, b). A number of well-defined landform groups occur in the area comprising sand beaches, yellow and grey dunes, dune pasture, raised shoreline terraces and fossil cliffs. They are interrupted by the seaward passage of the North and South Esk rivers, leading to the development of a river terrace suite between the dunes and the fossil cliffs along the North Esk course. The raised shoreline and river terraces are the most extensive relict landform groups and they consist largely of shingle (pebbles and cobbles with long axes between 4 and 256 mm). Fossil shingle is also found locally beneath the modern beach and dunes and is one of the dominant constituents of the till and mass-movement debris covering the cliff face. Modern shingle is areally restricted, occurring on the bed of the