Composition, origin and distribution of Dutch coastall sands between Hoek van Holland and the island of Vlieland.

C. CONCLUSIONSI. The coastal sands are predominantly reworked early-and middlePleistocene sands. North of Bergen reworked Saalian glacial sands, Meuse sands and Rhine sands of the S-association are present with probably a small admixture of sand of eastern origin and some Rhine sands of the AS-association. Leaching has resulted in loss of some hornblende and probably of some augite. The sands south of Bergen are mainly reworked Rhine sands of the S-association with an admixture of late-Pleistocene-Holocene Rhine sands. The proportion of Rhine sands of the AS-association is larger south of IJmuiden than between IJmuiden and Bergen.2. The existence of two types of sand of very different mineralogical composition near to each other without large scale mixing is due partly to the relatively recent (Holocene) conditions along the coast, partly to the general history of the sands. In pre-Saalian time, Rhine sands of the S-association and to a lesser extent, of the AS-association, were deposited in the northwestern Netherlands, together with some Meuse sands and, presumably, some sands of eastern origin. During the Saalian, ice covered the northern half of the Netherlands and sands of the A-association were deposited in that area. Since in post-Saalian time the Rhine had a mainly southern course, supplying some AS-type of sand to the southern coastal sands, virtually no sand of AS-type was supplied to the northern area.The mixing of the Saalian A-sands, Rhine sands of S-type and ,Meuse sands occurred mainly during the Eemian (by the sea), the Weichselian (by the wind) and the Holocene (during coastal erosion and the formation of inlets). Erosion during the Weichselian and the Holocene, and probably also during the Eemian, did not reach great depths, except locally in the tidal inlets, which explains the virtual absence of an admixture of sands of eastern origin, usually present at some 25-60 m or less below the Saalian deposits. During the Holocene the area north of Bergen remained land or shallow tidal flat until very recently, separated from the coastal sands farther south by tidal inlets. An inlet in the area of Bergen-Schoorl was filled up, around 3500 B.P., with sands from the north, whereas much later, during the early Middle Ages, an inlet at Bakkum-Bergen was filled up mainly from the south. After this inlet was closed more sand again came from the north, probably in connection with erosion of the coast north of Bergen and the formation of the tidal inlet between Huisduinen and Texel. Thus the zone of mixed sands between Bakkum and Bergen was formed, with lower feldspar contents near to the beach and higher feldspar contents in the most landward dunes. After a period of erosion, during which the sharp boundaries north of IJmuiden originated, the beach became stable between IJmuiden and Egmond, but continued to retreat farther north.3. Offshore the recent sediment pattern is mainly determined by the residual transport northwards by tidal currents of sand grains smaller than about 300 u, whereas along the beach the sediment pattern is mainly determined by sand movement at right angles to the coast, the residual transport (mainly of grains 200 u) parallel to the coast being small. Thus along the beach sharp boundaries are found between the various types of sand, whereas offshore the boundaries are diffuse.The beach sands north of IJmuiden are mainly locally reworked older Holocene sands, the boundaries between the various types coinciding with marked, older Holocene features. The sharp boundary at IJmuiden reflects a general change in grain size. The beach sands south of IJmuiden are mainly recently deposited sands moved shorewards from the sea floor.Offshore coarser sands occur off Hoek van Holland-IJmuiden and off Texel-Vlieland, whereas fine sands, containing an admixture of sand grains < 300 u transported northwards occur between IJmuiden and Den Helder. The areas of Hoek van Holland-IJmuiden and Texel Vlieland are areas of (slow) erosion, characterized by comparatively high amounts of dead mollusk shells, shell fragments and Eemian fossils. The area off IJmuiden-Den Helder is an area of (slow) deposition.4. Generally, offshore as well as along the beach and in the dunes, Al and Fe contents are comparatively low north of Bergen, high farther south. Along the beach and in the dunes there is also a difference in Ca and Mg content: it is low at km-piles 2-34, high at km-piles 44--92, with a gradual increases between km-piles 34-44.The distribution of Al and generally also of Fe is related to the origin of the sands. Al is contained in the feldspars; Fe, as considered here, is present in the coatings on the grains. Secondary (recent) deposition of iron occurs off Hoek van Holland-I]muiden and in low quantities ovcr the entire area of investigation. Secondary loss occurs under reducing conditions nearshore and, by leaching, under subaereal conditions. Leaching is especially evident in the sands north of Bergen.Only in the fine size fractions is the distribution of Ca and Mg (partly) related to the origin of the sands. In these size fractions Ca and Mg occur in detrital calcite grains and in a recent secondary admixture of organic carbonate grains (Foraminifera, Echinoid spines, Bryozoa). In the coarse size fractions ( > 200 u) Ca and Mg are mainly present as a recent secondary admixture of mollusk shell fragments.5.The secondary deposition of iron on the sea floor is related to the release of iron from sediment particles in the Rhine-Meuse estuary. This results in relatively high concentrations of dissolved iron in the estuary and in the nearshore water of the North Sea. Flocculation then leads to deposition of iron on the sea floor. On the beach, deposition of the iron floccules is prevented by strong turbulence.6.Shell fragments are formed mainly by fragmentation of the more fragile mollusk shells (of the species Abra alba, Abra prismatica, Angulus fabula, Angulus tenuis, Ensis sp.). This fragmentation is chiefly done by one species of bottom fish, Pleuronectes platessa, the plaice, which feeds on these mollusk species and cracks the shells. Mechanical action by currents and waves seems to be of minor importance.The distribution of the species with fragile shells indicates that the nearshore area with fine grained sediments is the main source of shell fragments. The distribution of fine shell fragments (315-400 u), however, does not show a nearshore maximum due to the subsequent landward transport of the fragments by the waves. Where the beach advances, as at km-piles 57-91, and also to a lesser extent where the beach is more or less stable (as at km-piles 37-53), shell fragments are accumulated on the beach. North of km-pile 37, shell fragments do not show a nearshore maximum partly because the coast is retreating, partly because this area only became marine in the Middle Ages.Offshore the relative amounts of shell fragments are high off Hoek van Holland-I]muiden and Texel-Vlieland, and low off I]muiden DenHelder where also sand moved northwards from the south, is deposited. The distribution of whole dead shells and coarse fragments> 1000 u is similar to the distribution of the living mollusks except of Hoek van Holland and of Texel-Vlieland where secondary accumulations have been formed by the removal of finer material.7.The distribution of magnesium generally follows 'the distribution of calcium but the Ca: Mg ratio is variable. This is due to: a, an irregular admixture of Echinoid fragments which have a higher Mg-content than mollusk shell fragments; b, secondary sorption of Mg from sea water on the shell fragments, and c, a relative increase of Mg during acid leaching under subaereal conditions.8.Summarizing, the difference in AI, Fe, Ca and Mg content between the southern and northern beach and dune sands is due to the combined effect of: a, the difference in origin; b, the pronounced effect of leaching in the northern sands; c, the fact that the northern sands became beach sands only during the Middle Ages whereas the southern sands have been beach or sea sands already since the Atlanticum; d, separation of the northern and southern sands by tidal inlets since the Atlanticum, which prevented the direct movement of southern sand towards the north; e, a generally slow residual movement of sand from south to north, and J, continuing erosion of the coast north of Bergen so that there is virtually no recent deposition of shell fragments on the beach.