stack

In computing, a method of storing data in which the most recent item stored will be the first to be retrieved. The technique is commonly called ‘last in, first out’.

Stacks are used to solve problems involving nested structures; for example, to analyse an arithmetical expression containing subexpressions in parentheses, or to work out a route between two points when there are many different paths.

stack

Stacks, such as this sea stack off the Dorset coast, are isolated upstanding rocks that are formed by erosion. Wave action pounds away at a rock barrier, such as a headland. First, it attacks weaknesses such as joints and faults, eroding a cave on the headland. The cave is deepened until it passes all the way through the headland to form an arch. If the top of the arch collapses then a stack is formed. Eventually the upstanding stack is eroded to form a stump.

The Needles and lighthouse, Isle of Wight. The Needles, at the western extremity of the island, are sea-stacks formed principally of chalk and flint; the unmanned lighthouse is on the furthest rock.

Isolated pillar of rock that has become separated from a headland by coastal erosion. It is usually formed by the collapse of an arch. Further erosion will reduce it to a stump, which is exposed only at low tide.