A sea stack is a pillar of rock found in the water close to a coastline. Sea stacks are common along many of the world's coastlines, and some have even become quite famous. Like other features found along shorelines, sea stacks are also in a constant state of flux, with new stacks emerging all the time while old ones disappear. Some sea stacks have been known to erode into very unusual and striking formations, making them popular subjects for photographers and painters.
A sea stack is caused by the natural erosion of headlands along the coastline. Typically, the ocean wears a hole through the headlands first, creating an arch which slowly expands over time as it erodes. Ultimately, the arch collapses, leaving a sea stack on one side and the headland on the other. Separated from the shoreline, the sea stack will slowly start to erode, ultimately melting away into the water or collapsing.
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Essentially, a sea stack is like a very small island, and in some cases, sea stacks have actually started out as islands which have been worn away. Many migratory birds use sea stacks for nesting and shelter, appreciating their isolation and relative safety. Sea stacks are also popular among rock climbers, since many such stacks pose interesting and fun climbing challenges.
The distribution of sea stacks in an area varies, depending on the kind of rock the headlands are formed from, ambient weather conditions, and the prevailing currents of the water. In some cases, an area may be littered with sea stacks made from very hard stone, while in other instances a shoreline has only a few stacks composed of soft, crumbly materials like limestone and sandstone. Since many headlands are formed from the former ocean floor, some sea stacks also reveal interesting fossil remains as they erode.
Some caution is advised when one is around sea stacks. It is possible for stacks to collapse unexpectedly, to the detriment of anyone who happens to be standing or boating nearby. When climbing sea stacks, it is a good idea to watch out for soft, crumbly rock which could give way under the weight of a climber, and to avoid especially narrow, spindly sea stacks as they can be very fragile. In addition, because sea stacks are used as nesting areas by birds, access to stacks may be restricted by conservation agencies in the interest of protecting the birds.