Shore gravels of Pleistocene Lake Bonneville in Great Salt Lake Valley, Utah, have yielded unusual pebbles which are, in shape, variants on the fundamental tetrahedron. Field investigations of stream channels, alluvial falls, and talus slopes in the canyons of the nearby Wasatch Mountains have shown that these distinctive pebbles originate as corner fragments broken from rhombohedral or sub-cubical joint blocks tumbling down from highly jointed bedrock outcrops onto talus slopes. It is suggested that the shape of such fragments renders them quite resistant to any appreciable distance of stream transportation. On the basis of these observations, it is believed that the occurrence of appreciable numbers of tetrahedroid pebbles in gravels and conglomerates strongly indicates the non-marine origin of such rock units.

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