A sand spit is a narrow, elongated ridge of sand or shingle that extends from the mainland into a body of water, such as a bay, an estuary, or the open sea. This coastal landform is created entirely by the accumulation of sediment. Sand spits are significant features in coastal geography, often protecting quieter water areas and influencing local ecosystems. They represent a balance between the forces of sediment movement and the coastal environment.
Physical Description of Sand Spits
A typical sand spit appears as a low-lying, linear accumulation of material, with one end firmly anchored to the continuous coastline. The composition is generally a mix of sand, which is the most common material, or coarser shingle and gravel, depending on the sediment source. This ridge has two distinct sides: the exposed side and the sheltered side.
The exposed side faces the open water and receives the full force of incoming waves, often featuring a steeper profile. In contrast, the sheltered side is protected by the spit itself, leading to calmer, shallower water. This quiet environment often allows for the development of mudflats and salt marshes, as fine silt and clay settle out of the water column. The end attached to the land is called the proximal end, while the unattached, projecting tip is referred to as the distal end.
The Dynamic Process of Spit Formation
The formation of a sand spit is driven by longshore drift, which is the movement of sediment along a coast. This occurs when waves approach the shore at an oblique angle, causing the water to wash up the beach face (swash) at a slant. Gravity then pulls the water and sediment directly back down the beach (backwash) perpendicular to the shoreline. This continuous zigzag movement effectively transports vast quantities of sand and gravel in a consistent direction along the coast.
A spit begins to form when longshore drift encounters a sharp change in the coastline’s orientation, such as the entrance to a bay or an estuary. At this break point, the current carrying the sediment load spreads out into deeper water, loses velocity, and can no longer transport all of its material. The sediment is then deposited in the quieter water, initially forming a submerged feature that eventually builds up above the high-tide mark to become a spit.
The spit continues to grow outwards following the direction of the dominant longshore drift until the growth is counteracted by other forces. These opposing forces can include strong river currents or deep-water wave action, which can halt further deposition. Wave refraction around the distal end is also a factor, causing waves to bend into the sheltered water, carry sediment, and cause the tip of the spit to curve inwards toward the land.
Variations and Related Coastal Landforms
The basic spit structure can evolve into several distinct geographical variations based on localized conditions. One common variation is the recurved spit, sometimes called a hooked spit, which has a prominent inward curl at its distal end. This hook shape results from secondary wave patterns or changing wind directions that cause wave refraction to push the tip toward the sheltered area.
When a spit continues to grow across the mouth of a bay until it connects to the land on the other side, it forms a landform known as a bar or barrier beach. This connection effectively seals off the bay from the open ocean, creating a sheltered body of water called a lagoon or marsh behind it.
A different development occurs when a spit extends and connects the mainland to a pre-existing offshore island. This connecting ridge of deposited sediment is specifically termed a tombolo.