Philorth valley, Fraserburgh - locality, Cainozoic of north-east Scotland: Difference between revisions
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From: Merritt, J W, Auton, C A, Connell, E R, Hall, A M, and Peacock, J D. 2003. Cainozoic geology and landscape evolution of north-east Scotland. Memoir of the British Geological Survey, sheets 66E, 67, 76E, 77, 86E, 87W, 87E, 95, 96W, 96E and 97 (Scotland).
Philorth valley, Fraserburgh
The concealed sediments within the Philorth valley, on Sheet 97, include a sequence of estuarine deposits and interbedded peat. This sequence provides important evidence of the pattern of relative sea level change in north-east Scotland during the Holocene. Because of the marginal setting of the Philorth valley relative to the centre of postglacial isostatic rebound in the western Highlands, these deposits preserve a more detailed record of sea-level movements than is recognised from most other coastal localities (Smith et al., 1982; Smith, 1993). The Water of Philorth is a small stream that drains north-eastwards to reach the coast 3 km south-east of Fraserburgh (P915374). The estuarine deposits there are concealed beneath brown silty clay and blown sand at the northern end of the valley (NK 011 635), west of Milltown. The buried sequence records changes in relative sea level during the middle and late Holocene, including a transgressive episode not recognised elsewhere in Scotland. Mapping, levelling and coring by Smith et al. (1982), proved a succession of sands and gravels overlain by peat, with interbeds of grey sand and micaceous sandy silt, beneath the brown silty clay (P915311).
Pollen from the top of the peat above the grey sand layer, in the overlying micaceous sandy silt and in much of the peat above it, indicates an increase in the occurrence of oak and alder; the silt is associated with high values of pine and oak. The top of the peat and the overlying brown silty clay yielded pollen indicating birch-oak woodland, with alder and local freshwater aquatic communities.
The age of the micaceous sandy silt is constrained by both the pollen and the radiocarbon dates, which indicate deposition during the middle Holocene (P915358). The association of Plantago maritima pollen, together with the increase in representation of pine and oak, is characteristic of a littoral depositional environment (Traverse and Ginsberg, 1966) and indicate that the silt is of marine-estuarine origin. Smith et al. (1982) suggest that this deposit represents local expression of the Main Postglacial Transgression (when sea level was relatively higher than at present). They interpret the radiocarbon evidence as showing that the transgression was underway in the area by 6300 ± 60 14C years BP and that it culminated after 6096 ± 75 14C years BP. Surface peat growth had recommenced by 5700 ± 90 14C years BP. These dates are somewhat later than those proposed for this event from sites farther to the south (Smith, 1993) and may be taken as evidence that the transgression was diachronous and resulted in the formation of a time-transgressive shoreline. The brown silty clay present at the surface began to accumulate at about 4760 ± 60 14C years BP as the result of a second rise in relative sea level, again to above present OD.