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Te xt ural dwersity of fluvial de po sus

only at Podgrodzie. These deposits show a twofold division (Fig. 14D). The first group consists of coarser deposits a (Mz — 1.7 — 3.1 0) being moderately and poorly sorted. They accumulated in channels interrupting the alluvial fan surface.

The second group b is composed of poorly and very poorly sorted deposits the Mz values of which rangę from 3.3 0 to 6.0 0. These raay be subdivided into two parts. The first one b_x includes deposits the Mz of which varies from 3.3 0 to 4.7 0. Sorting is moderate and poor. The decreasing mean diameter indicates that their depositional environment showed tendencies toward decreasing current velocrties. It is likely that their accumulation took place dose to the channel on the fiat alluvial fan surface. This sediment may correspond to a levee deposit. The second part b₂ includes sediments with Mz of between 5.0 and 6.0 0. Sorting is poor and very poor. These properties are typical of deposits accumulated in a non-current environment. It seems that their sedimentation from suspension took place in depressions extending outside the levees. The extent of the deposits discussed is greater than that of the coarser sediments b_v

The flood-formed deposits in the Vistula channel and on the floodplain c show smali variations in grain size and sorting. There is a “region of overlap" between those deposits and the channel deposits having a given Mz (Fig. 14E). Deposits laid down by the Wisłoka appear to be similarly clustered, although some of them are placed into the group of fine and very fine grains which display characteristics of overbank sediments.

Because of channel incision d u ring the last century (Klimek 1974; Alexan-drowicz et al. 1981), deposits forming the modern gravel bars a in both rivers are marked by Mz greater than that of the coarsest fossil deposits. Conseąuent-ly, the “armoured” channel tends to shift only slightly at high discharges. Sediments forming the new sandy-gravelly bar b display characteristics of channel deposits.

THE CHARACTERISTICS OF DIFFERENT TYPES OF FLUVIAL SEDIMENTS BASED ON ABRASION INDICES

In order to characterize the different types of fluvial deposits, analysis of quartz grain abrasion also was madę. The grain shape and abrasion depend upon many external factors, such as parent materiał, source of supply and distance of grain transport. They also depend upon internal factors, i.e. the modę of transport, selection during transport and sedimentation, and type of channel bed (Gradziński et al. 1986). The aim of the present analysis of plentiful data which were related to different depositional subenvironments of known ages is to recognize factors and their effects on a river.

The characteristics of deposits is based on scatter diagrams of abrasion index Wo versus standard deviation <r₀. This analysis madę in the fractions 0.75 — 0.5 mm and 1.02 — 0.75 mm shows that the coarser grains are better abraded (higher W o values) and morę homogeneous (Iow values of the homogeneity index). It appears that the coarser the fraction, the greater is both the abrasion degree and the homogeneity of the deposit examined. This rule