<span>Igneous or metamorphic. Depending on the energy of the river, they could be anything from large, rounded cobbles or pebbles (as per classification of the Udden-Wentworth Scale), or sand and gravel as identified under other systems. Or angular clasts that haven't been rounded by moving along the bed. Note that the Udden-Wentworth is usually applied to sedimentary features, but can also be used in classifying the *size* of any rock. </span>
