Why do sediments have angular or rounded fragments?

Answer 1

Angular sediments are usually young or "immature" sediments which have only recently been weathered from a source rock. They have not been exposed to corrosive fources long enough to become worn down. These sort of sediments are common in glacial areas or areas high in a river catchment near their original location.

Rounded grains suggest extended periods of transport and mechanical weathering, which chip away the sharper edges of the immature sediments leaving them worn dow. a common example of this is a desert sand, which has been exposed to errosive forces for a long time, being blown around the desert, giving them a rounded appearance.

Think of a broken piece of glass. In its "immature form, it has sharp edges and smooth faces. If you throw it in the ocean on a beach, and give it time, the surf and sand will weather it smooth, leaving you with nice smoth beach glass, a mature sediment.

Answer 2

Sediments can have angular fragments if they have not been transported far from their source, maintaining their sharp edges. In contrast, sediments with rounded fragments typically result from prolonged transportation and abrasion, smoothing their edges over time. Environmental factors such as water currents or wind strength can also influence the shape of sediment fragments.