Studies of alluvial gravel demonstrate that the percentage of a given rock type in the gravel varies with the size distribution of the particles in the sample. The variations are large and account for differences of 25 percent or more between lithologies of unsized samples of the same formation. Accordingly, most pebble counts of alluvial gravel should utilize restricted grade sizes determined by sieving or other methods of accurate size measurement. Studies of the coarse fraction of Kansan till from northwestern Missouri and northeastern Kansas indicate that the lithology of till also varies with the mean diameter of the particles in the sample. The variations are, however, less than one-fifth as great as the variations found in alluvial gravel. This contrast in size-lithology variation may represent a fundamental difference between till and alluvial gravel. Some restriction of grade size is desirable for pebble counts of till. Sorting by visual estimation of size is probably accurate enough for most routine descriptions of till.

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