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A sedimentological study of Mor Bras, Brittany (France) The present contribution belongs to a submarine research project in progress on the nearshore continental shelf of the Bay of Biscay between Quiberon Peninsula and Les Sables d'Olonne, and describes the sedimentary features of Mor Bras, which were studied by dredgings and echo-soundings (Fig.1). Mor Bras is a submarine basin, sheltered by a rocky shoal running from Quiberon to Hoedic Island. This isolation explains the seasonal diminution of salinity consecutive to fluviatile floods, and is the chief cause of the formation of a calm water mass surrounded by strong tidal currents (Fig.2). The major characteristics and the distribution of sediments are shown on sedimentological sections with figuration of pelite-sand-pebbles ratio variations, bathymetry, calcium carbonate and organic carbon contents (Fig.4), on a map giving the calcium carbonate contents (Fig.5), and on a general map established according to new principles (Fig.6), in which the figuration of the sedimentary supply is detailed. The rocky margins, Pleistocene pebbles, angular stones and gravels are the basic representation; the sandy and pelitic contents are respectively mapped by isopleths and flat tints. Special conventional symbols can express the sorting of sediments (Trask Sorting Index, for instance) or the mineralogic and biologic composition of a few sands. The distribution of sediments is regular, displaying a pattern of concentric areas in which the median diameter becomes finer and finer from the rims to the central part of the basin: this is classic in numerous land-locked seas and bays. Mor Bras includes two kinds of sedimentary environments, the geographical distribution of which is controlled by direction and strength of tidal currents (Fig.7): 1. ( 1 ) The detrital rim, devoid of muddy sediments (pelitic ration below 10%) due to the relative velocity of currents, where the skeletal fraction plays the predominant part. In this belt (mostly around the Bay of Quiberon), the following areas are described and analysed: ( a ) the rocky hard bottom, which is sometimes exposed, and in other places covered by Laminaria or by migrating coarse calcarenites; ( b ) the “coralligenous” areas colonized by photophile calcareous Algae, the Lithothamnion or “maerl”; ( c ) the infra-littoral beaches composed of fine sand off the tomboles of Quiberon and Le Croisic; ( d ) and the main part of the sandy and calcarenite regions, moulded by tidal currents running across the Strait of La Teignouse, such as the Bank of Quiberon, characterised by a complex pattern of sandy ridges (Fig.9, 10). 2. ( 2 ) In the inner part of the basin, where tidal currents are much weaker (below 1 knot), the finer particles can settle down: the mud deposition shows very different aspects depending on the local environments (Fig.8). In the Bay of Quiberon off the Vilaine estuary (Fig.11) the silt particles (median diameter between 1–10 μ) are restricted to some depressions separated by enbankments of muddy sand: depressions off the entrance of the Gulf of Morbihan and on both sides of the submarine delta formed by deposition of the Vilaine solid discharge. In the Bay of Vilaine, mud with a small pelitic ratio is predominant (Fig.12). The fine part of this mud consists chiefly of particles of silt size and its organic matter content is intermediate between the percentages in the mud on the shelf and those in the tidal marshes. The source of pelitic supply is continental with regard to the Vilaine river and perhaps to the Loire river. More definite conclusions perhaps can be reached by chemical analyses of the sediments. Mor Bras is a sediment trap and an environment mostly controlled by regional or local geographical conditions.

Summary The investigation attempts to ascertain rates and amounts of soil erosion from a small unvegetated area (120m×15m). Measurements from markers embedded in the soil indicate a removal of 10 mm per annum on 5°C slopes, 32 mm per annum on 45°C slopes, and 43 mm per annum on 62°C slopes. A sediment trap indicated that in a week when there was 56·8 mm rain, approximately 60 g/sq m was eroded. Results of the work are used in an explanation of the eroded soils of a much larger area, known as the Lower Swansea Valley.

Agua Hedionda Lagoon is near Carlsbad, CA, and is near the middle of the Oceanside littoral sedimentation cell. Since its initial dredging in 1954, the lagoon has become a coastal sediment trap that must be dredged almost yearly. The purpose herein is to show the rate of sediment deposition in the lagoon from 1955 to 1961.

The turbidity maximum near the head of the Chesapeake Bay is produced primarily by the local resuspension of bottom sediments, and by the estuarine "sediment trap" which is formed in the upper reaches of the estuarine circulation regime by the net nontidal circulation.

Mixed terrigenous and pelagic sediment of Eocene age was recovered from the deep-sea floor 250 km east of Hawaii. These samples indicate Eocene terrigenous sedimentation from a well-developed ridge source and imply a post-Eocene development of the Hawaiian Moat as a sediment trap. Another core that also contained Eocene sediment, mixed however, with Oligocene and Miocene materials, was obtained from the flank of the Hawaiian Ridge, 100 km south of Kauai.