The definition and scope of Ordovician genera of agnostid trilobites are discussed, and the generic affiliation of Ordovician species and well-illustrated material referred to in open nomenclature is assessed. Some 198 Ordovician species plus 25 species of uncertain latest Cambrian/earliest Ordovician age are enumerated, representing 24 genera and subgenera, of which Geragnostus (Novoagnostus) n.subg. and Lotagnostus (Semagnostus) n.subg. are new. Geragnostella is ranked as a subgenus of Geragnostus. The species of Arthrorhachis are tentatively divided among a tardus species group and an elspethi species group. Species of Dividuagnostus are divided among a sensu stricto group and an early species group. Pseudorhaptagnostus Lermontova, 1951 is restored as senior synonym of Neoagnostus Kobayashi, 1955. It is concluded that Ordovician agnostids were largely adapted to temperate and colder water habitats.

The stratigraphy of the Neoproterozoic Dalradian Supergroup has been formalised at group, subgroup and formation level, based largely on mapping in the SW Highlands of Scotland. Elsewhere, there has been a tendency for the lithostratigraphy to be fitted into this framework, implying that the lithostratigraphy represents basin-wide changes in depositional environment. The pitfalls of this ‘layer-cake’ approach are illustrated by interpreting the geology of the Gleann Fearnach to Glen Shee area of the Central Highlands, using the published maps of the adjacent regions. Attempts to correlate units into the Braemar area in the NE and into the Pitlochry area in the SW show considerable mismatch between the maps covering these areas, which has in the past led to invocation of geometrically unlikely arrangements of faults and slides to explain the lithostratigraphical distribution.

The present study is the result of a different approach adopted during remapping of the Gleann Fearnach to Glen Shee area. Some basin-wide events, here suggested to be associated with key sequence stratigraphic surfaces, were identified. These principally relate to the deposition of Appin Quartzite, Boulder Bed and Ben Lawers Schist precursor sediments. Between these reference units the stratigraphy has been determined by detailed field mapping, and variation from the standard stratigraphy explained in terms of both the position within the depositional sequence and palaeogeography during deposition. This has led to an integrated solution which works on both sedimentological and structural grounds. There are local implications for the stratigraphy and structural geology elsewhere in the Scottish Dalradian, but, more importantly, it is an approach which must be more widely applied in order to make wider correlations beyond the British Dalradian to the rest of the Laurentian margin.

Low altitude sandy plains (machair) are a common feature of the Atlantic coasts of the Outer Hebrides. They formed as a result of shoreward movement of material consequent upon a rise in Holocene sea levels. A number of earlier, partially inconclusive, investigations into machair evolution proposed that fuller understanding could arise from an examination of inter-tidal organic deposits, in so far as these could provide a fossil proxy record of the processes involved in machair formation. Study of a series of inter-tidal peats sites located on a beach platform at Borve, island of Benbecula, provided both spatial and chronological evidence for sand movement as well as furnishing new data on vegetational and environmental history. The pollen diagrams show a flora in which Calluna vulgaris (heather) and Poaceae (grass) are frequent dominants, while such arbroeal taxa as Betula (birch) and Corylus avellana-type (cf. hazel) are notable. The existence of birch–hazel woodland for the period c. 8855–6190 BP might conceivably have had a retarding effect on sand movement. Later cycles of sand movement would not have met such obstruction with the consequence that sand mobility and machair plain evolution could have been faster. Sand arrival at the seaward site (Borve 3), is dated to c. 6735 BP and it continued until c. 6045 BP, after which it slowed before increasing again from around 5990 BP. This sand incursion produced a machair plain over the Borve sites as part of a continuous, but variously paced, marine and aeolian process. The presence of charcoal from c. 6860 BP, with a major increase in charcoal to pollen ratios by 6190 BP, may suggest that natural or intentional burning of the vegetation cover of the machair occurred in Mesolithic times; if the burning was anthropogenic in origin, then it pushes the possible involvement of humans in machair destabilisation to a time long before the previously proposed Neolithic period. A wet slack deposit, dated to 3400 ± 70 BP, indicates a time when sand movement overwhelmed the area around the landward site (Borve 1) and when the rate of sand movement was likely to have been impeded. The data from Borve suggest that evolution of machair landforms occurs at a slow, variable rate and that there is considerable long-term stability.

Transactions of the Royal Society of Edinburgh: Earth Sciences 87, 95–103, 1996