24 SEDIMENTOLOGY OF THE YTHAN ESTUARY, BEACH AND DUNES, NEWBURGH AREA N. H. TREWIN PURPOSE The object of the excursion is to examine recent sedimentological features of the Ythan estuary and adjacent coast. Sedimentary environments include sheltered estuarine mud flats, exposed sandy beach and both active and stabilised wind blown sand dunes. Many of the sedimentary features to be described are dependent on local effects of tides, winds and currents. The features described are thus not always present, and the area is worth visiting under different weather conditions particularly during winter. ACCESS Most of the area described lies within the Sands of Forvie National Nature Reserve and all notices concerning access must be obeyed, particularly during the nesting season of terns and eider ducks (Apr.-Aug.) when no access is possible to some areas. Newburgh is 21 km (13 miles) north of Aberdeen via the A92 and the A975. Parking for cars is available at the layby by locality 1 at [NK 006 2831], and on the east side of Waterside Bridge for localities 2-8 (Fig. 1). Alternatively the area can be reached by a cliff top path from The Nature Reserve Centre at Collieston and could be visited in conjunction with Excursion 13. Localities 9- 10 can be reached from the beach car park at [NK 002 247] at the end of the turning off the A975 at the Ythan Hotel. There is a single coach parking space at the parking area at Waterside bridge, but the other parking areas are guarded by narrow entrances to prevent occupation by travellers with caravans. Walking distance on paths and shore is 9 km for localities 1-8 starting at locality 1 or 6 km starting at Newburgh Bridge. Low tide is essential for examination of the estuarine mudflats and channel. The dunes and beach at localities 9-10 (2 km walking), and locality 11 (7 km walking), can be examined as an excellent alternative when the Nature Reserve is closed. Visitors are strongly advised against venturing onto the mudflats for their own safety and to prevent disturbance of areas under scientific study. Binoculars will be found useful, and wellington boots are advised for the estuary. The area is covered by O.S. 1 :50,000 sheet 38. INTRODUCTION The excursion is designed to illustrate features of sedimentological and ecological interest from the geological viewpoint. Details of fauna and flora have thus been kept to a minimum. Fig. 1. Locality map of the Ythan Estuary. Intertidal sediments are indicated in yellow, and the extent of sand dunes in pale brown. In the north of the Forvie reserve the dunes are all vegetated. ITINERARY Locality 1. Inner estuary [NK 006 284] The northerly layby overlooks an area of sand and mudflat on the east side of the river. The mudflat is crossed by a small tributary stream and backed by an area of saltmarsh. Behind the saltmarsh is an overgrown river cliff indicating that at one time the main channel was close to the western bank. Saltmarsh. The saltmarsh area is only flooded on rare occasions and is above normal high tide level. The top of the saltmarsh is relatively dry and vegetated with a wide variety of plant species when compared with locality 2. Only a few inlets and pools occur within this marsh area; the pools are normally water filled but have mud floors with polygonal mud cracks indicative of periods of desiccation. Only a sparse fauna is present but some pools contain the amphipod Corophium and shore crabs. Strand line material occurs near the edge of the marsh and includes marine elements such as crab shells, seaweed with barnacles and even jellyfish which have been brought up river on the flood tide; also present is vegetation brought down by the river from upstream. The edge of the saltmarsh is broken and degraded and minor erosion appears to be taking place between the layby and the tributary stream. Beyond the tributary stream the character of the saltmarsh top changes due to the presence of a thin covering of blown sand which is best seen 100 m north of the tributary where a small beach is developed and wind blown sand overlies saltmarsh sediments, and bears a different vegetation from the normal saltmarsh top. Tributary stream. In 1985 the tributary stream had a sand and gravel bed which usually showed well developed linguoid ripples with gravel visible in the troughs and black heavy minerals concentrated on the ripple crests. However in March 2008 (see Fig. 2 below) there was no sand present and the stream was eroding marsh deposits. Fig. 2 Stream emerging from saltmarsh and crossing mudflat towards channel at Loc 1 The stream bed is soft! At the first bend in the tributary as the stream crosses the mudflat the stream has eroded a small cliff c. 30 cm high in the mudflat sediments which are black and reduced below the surface but retain a brown oxidised zone around burrows which allow water circulation. On the inside of the bend a small point bar of rippled and laminated sand is in the process of deposition. Faster flow in the straight shallow channel downstream of the bend results in the formation of isolated sand ribbons rather than ripples on the channel floor. Upper beach. At the top of the beach zone a narrow sandy zone is developed which has scattered angular pebbles on its surface in the area near the layby. The surface is usually smooth but small parallel crested ripples are sometimes present. Concentrations of shells of the small gastropod Hydrobia frequently occur at the top of the beach in hollows at the marsh edge. Mudflat. The sandy upper beach grades into the mudflat away from the saltmarsh edge. Increase in mud and water content of the sediment is noticeable! The fauna of the mud surface is dominated by Hydrobia (in summer) and the small U-shaped burrows of the amphipod Corophium , which in places are found right up to the saltmarsh edge. The only bivalve living in this area is the shallow burrower Macoma balthica . The surface sediment is extensively bioturbated. This area is 4 km from the open sea and the marine fauna consequently reduced in comparison with localities 2 and 3. The bivalves Cardium (cockle) and Mytilus (mussel) do not live in this area, which is near the upper limit of barnacle and seaweed growth in the estuary. Accumulation of sediment in this area has been taking place (since 1978) on the high area of the sandflat adjacent to the tributary stream. In 1985 this area was in the process of colonisation by grass and algae, with some areas covered in summer with a resistant mat of filamentous green algae and with clumps of grass established on the sand surface. Partial drying of the area occasionally produced incipient polygonal mudcracks at the end of the high area near the tributary stream. Little has changed in the last 30 years, if anything the grass has been reduced to a few smaller patches, and saltmarsh has not become established. Main river channel. The main channel has a floor of fine gravel with migrating sand bars consisting of small flat dunes with a wavelength of a few meters which are associated with ripple marks. At low tide the tops of the dunes are generally exposed, but not readily accessible. Movement of the sand bars within the channel system causes changes in the channel position from time to time and probably influences deposition on the adjacent mudflat. Locality 2. Middle estuary [NK 003 271] Beside the parking area at [NK 004 270] on the east (upstream) side of Waterside Bridge is an area of saltmarsh connected to the estuary shore by a small tidal creek which comes to within a few metres of the main road. The top of the salt marsh is wetter than at locality 1 and has a less mature flora, with sea plantain and sea pink in evidence. In the creek salinity is sufficient to support a fauna including the lugworm Arenicola whose castings can be seen on the creek floor. The absence of strong currents in the creek allows the castings over the exhalent arm of the V- shaped burrow to build up as a mound while a depression is created at the inhalent arm of the burrow. A 'pit and mound' structure is thus built up by the activity of the worms on parts of the creek floor. Corophium is abundant in the creek together with ragworm (Nereis), shore crabs and the small gastropod Hydrobia. The marsh top is flooded by the highest tides. Isolated pools on the marsh surface also have a fauna including Nereis , shore crabs and abundant Corophium. The burrowing activities of the latter can be watched in the shallow, clear water of the pools. Drying out of the larger pools in summer leaves a mud floor with tracks of birds and crabs and polygonal cracking of the mud takes place. Smaller pools which appear to maintain a permanently damp substrate but do not have standing water, are covered with a green wrinkled algal and bacterial mats. On the adjacent shore the increase in abundance of barnacles and seaweed attached to pebbles is noticeable, and in summer green fleshy plants of glasswort (Salicornia) colonise the upper beach. The numerous articulated Cardium shells lower on the shore are derived from a living population in this intertidal area. Mytilus shells are also present in this area derived from sparse living populations which occur up to 300 m upstream of this locality, particularly on the western bank.