Final answer:
Longshore drift on a beach occurs when waves approach the shore at an angle, creating a longshore current which moves the sand along the coastline. The movement of sediment can form cross-bedding structures and can be influenced by shoaling and overwash events.
Step-by-step explanation:
Longshore drift of sand on a beach occurs when waves approach the shore at an angle. This creates a longshore current, which is the dominant current in the nearshore zone and typically runs parallel to the shore. As waves hit the shore at an angle, they carry sand and sediment up the beach at an oblique angle.
When the water recedes back into the ocean, it pulls the sand perpendicular to the shore, due to gravity. This zig-zag pattern of sediment movement causes longshore drift, gradually moving sand along the coastline.
Factors like wave energy, which increases due to shoaling as waves reach shallow water, contribute to the intensity of longshore drift. When sand and sediment are carried and deposited by such currents, they can form cross-bedding patterns in the geological record.
These are layered structures that indicate the direction of water or wind flow, with the steeper side of the layers pointing in the direction the sediment was moving. Additionally, overwash events from storm waves can transport sand over coastal barriers, contributing to longshore drift.