(63) Answer:

At the younger stage of a stream different size and shapes of hard rock particles are added to the flow due to rapid mechanical weathering action. These particles have sharp edges at the very beginning of its formation. However, intensive mechanical weathering action at the younger stage of a river causes to remove sharp and irregular edges of such partials mentioned above with the long distance of their travel in the flow path. This is mainly due to the action of mechanical erosion at the early stages of the river flow. Finally, mechanical weathering of the long distance transported particles cause to produce well rounded particles with smooth surfaces.