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Abstract

Paleocurrents were determined from gravel fabrics measured on the surface and in vertical sections of proximal reaches of a valley train in the Yukon, Canada, and an outwash plain in Iceland. They agree closely with the mean orientation of surface channels and with the long axes of braid bars where data are grouped for a sufficiently large area. Orientation was measured as the direction and angle of dip of AB, the maximum projection plane of discoidal clasts. Samples of 30 to 40 clasts gave consistently significant individual vector means, whereas 20-clast samples appear to be sufficient if data from several samples are grouped. Coarser gravels tend to have a higher degree of preferred orientation than finer gravels.

The dip of AB averages 26 to 33 degrees upstream, giving a fabric which is consistent with deposition on subhorizontal surfaces. It is also consistent with an internal bar structure dominated by poorly defined horizontal bedding or massive gravel, with rare cross-stratification. Gravel braid bars are regarded as primary bedforms that are stable under flood conditions when all the bed material is in motion. Horizontal bedding is typical of most glaciofluvial gravels, because even at flood stage the flow is spread over a wide tract and the water is too shallow for the bars to develop slip faces. This contrasts with some Pleistocene gravels deposited by floods of exceptional depth and discharge, which formed giant braid bars with abundant large-scale cross-bedding (Bretz, Smith and Neff, 1956; Malde, 1968).

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