Geography · Ch 12 — Landforms and their Evolution
Bars, Barriers and Spits
Bars, Barriers and Spits
The satellite image shows a stretch of coastline where the Godavari river meets the sea, and the focus is on the shoreline itself rather than the inland delta. What stands out is a long, narrow strip of sand that has built out from the shore into the water, curving gently as it extends. This is the spit — a depositional ridge of sand and shingle that has grown outward from the land, not across the mouth of a bay but attached to a headland. The headland acts as the anchor point, the fixed end of the spit, while the free end trails off into the sea, following the direction of the prevailing waves and longshore drift.
The image is a top-down view, so you see the spit as a pale, curved line against the darker water, with the delta's distributaries and sediment-laden water nearby. The key teaching point is that a spit is not a random sandbar — it is a barrier bar that has become "keyed up" to one end of a bay or to a headland. In the textbook's scheme, an offshore bar lies parallel to the coast in deeper water; when it grows and becomes exposed, it is a barrier bar; and when that barrier attaches itself to land at one end, it becomes a spit. The Godavari delta image is a real-world example of that final stage, showing how a depositional feature can extend from a natural anchor point like a headland. …
A coastline is not a fixed line. It is constantly being reshaped by waves, currents and the material they carry. One of the most visible results of this work is the family of coastal ridges built from sand and shingle — bars, barriers and spits. These forms all begin the same way: as a ridge of loose sediment deposited in the sea, roughly parallel to the shore.
The ridge that forms in the offshore zone, lying approximately parallel to the coast, is called an off-shore bar. When further sand is added to it and it becomes exposed above the water, it is termed a barrier bar. These two features commonly form across the mouth of a river or at the entrance of a bay. In other words, they tend to grow where a river meets the sea or where a bay opens into the ocean — precisely the places where sediment supply is rich and wave energy drops.
Sometimes a barrier bar does not stretch all the way across the bay mouth. Instead, it gets keyed up, or attached, to one end of the bay. Such a feature is called a spit. Spits may also develop attached to headlands or hills that jut out into the sea. So a spit is essentially a barrier bar anchored at one end, extending into open water.
As barriers, bars and spits grow at the mouth of a bay, they gradually extend further and further, leaving only a small opening between the bay and the sea. When that happens, the bay is cut off from the open ocean and develops into a lagoon — a shallow body of salt water separated from the sea. The lagoon does not stay open forever. It gets filled up gradually by sediment coming from the land, or from the beach itself, with wind aiding the process. Over time, the lagoon is replaced by a broad and wide coastal plain.
The sequence is: off-shore bar → barrier bar (when exposed) → spit (when keyed to one end of a bay or headland) → lagoon (when the bay mouth is nearly closed) → coastal plain (when the lagoon fills with sediment).
The section then turns to a different agent of erosion — wind — and its role in deserts. Storm winds are very destructive. Wind works on the land through three processes: deflation, abrasion and impact. Deflation is the lifting and removal of dust and smaller particles from the surface of rocks. In transportation, sand and silt act as effective tools that abrade, or wear down, the land surface. Impact is the sheer force of momentum that occurs when sand is blown into or against a rock surface — it is similar to a sand-blasting operation. …
A ridge of sand and shingle formed in the sea, in the off-shore zone, lying approximately parallel to the coast is called an off-shore bar. An off-shore bar exposed due to further addition of sand is termed a barrier bar. Off-shore bars and barriers commonly form across the mouth of a river or at the entrance of a bay; sometimes such barrier bars get keyed up to one end of the bay, when they are called spits -- spits may also develop attached to headlands/hills. The barriers, bars and spits at the mouth of a bay gradually extend, leaving only a small opening of the bay into the sea, and the bay will eventually develop into a lagoon. The lagoons get filled up gradually by sediment coming from the land or from the beach itself (aided by wind), and a broad and wide coastal plain may develop, replacing a lagoon. Coastal off-shore bars, barriers, beaches, beach dunes and mangroves (if any) offer the first buffer or defence against storm or tsunami waves by absorbing most of their destru …