Sedimentary structures

Sedimentary structures can be of either physical (e.g. wave action) or biological (e.g. disruption of sediments by animals) origin.

Sedimentary structures are very important as they provide us with information on the palaogeography and palaeoclimate of the areas in which they occur. They can also indicate the direction of palaeocurrents of rivers and seas.

Physical structures

The movement of sand grains in a current creates ripples and dunes on the stream bed. These are known as bedforms. Ripples are the low narrow ridges that are separated by wider troughs. Physical structures form on sand dunes, on underwater sandbars in rivers and streams, and under the waves at beaches. They come in a wide variety of shapes and sizes which are characteristic of the currents that form them:

• Asymmetrical ripples: ripples that have a gentle slope upstream and a steep slope downstream.
• Cross-bedding: inclined bedding and commonly forms in alluvial environments.
• Potholes: rounded depressions caused by swirling currents and eddies.
• Mud cracks: formed by evaporation on mudflats or in shallow lakes.

Biological structures

These are made by living things and include:

• worm burrows (usually in soft-sediment, particularly at high tide levels along beaches)
• crab burrows (particularly those of hermit crabs)
• tubeworm colonies
• animal tracks on soft sediment