Abrasion is the process of rocks rubbing against each other and against the streambed. This constant grinding action wears down the sharp edges and corners of the rocks, gradually smoothing them out.
Attrition is the process of rocks colliding with each other. As rocks are carried along by the current, they bump into each other, chipping off pieces and breaking down their angular shapes.
Here's how these processes work together:
1. Initial angular rocks: Rocks initially transported by streams are typically angular, with sharp edges and corners.
2. Abrasion and attrition: As the rocks are carried along the streambed, they are subjected to abrasion and attrition.
3. Smoothing and rounding: The continuous grinding and collisions gradually wear down the sharp edges and corners, resulting in smoother and more rounded rocks.
4. Further transportation: As the rocks continue to be transported, they are further smoothed and rounded by the ongoing abrasion and attrition.
The degree of rounding depends on several factors:
* Distance traveled: The longer the distance the rocks are transported, the more rounded they become.
* Stream velocity: Faster currents cause more collisions and grinding, resulting in faster rounding.
* Rock hardness: Harder rocks are more resistant to abrasion and attrition, and therefore take longer to become rounded.
Overall, the smooth, rounded shape of cobbles and pebbles is a testament to the relentless forces of nature at work, shaping the landscape over time.
