Downstream changes in shape of pebbles of rhyodacite and sandstone in the Tambo River drainage basin can be related to four main processes: abrasion, shape sorting, dilution and breakage, but variations in mean pebble weight and mean altitude of source may also have some effect. Quantitative evaluation of factors causing downstream changes in shape of pebbles in the size class 32-64 mm shows that the relative effects of processes can be estimated by using a linear, multiple-correlation regression model. Highly significant multiple correlations are obtained with rhyodacite due to the isotropic nature of its pebbles. With sandstone the technique is not so successful.

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