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A promontory is a chunk of land which is significantly higher than neighboring land or water. This geographical feature can be found in large and small forms all over the world, and promontories have long been features of note for humans, since they provide an excellent view of the surrounding area. On the ocean, a promontory may be known as a headland, while on land a promontory could sometimes be confused with a steep hill. In either case, the process of formation is the same.
Erosion is the driving force behind the formation of promontories. Wind, rain, and gravity eat away at the surface of the Earth over thousands of years to shape features like promontories and valleys, among many others. Typically, the rock in a promontory is much harder than surrounding rock and sediment, which are worn away more quickly to leave a hard spine or ridge which becomes a promontory. Differences in rock composition can be caused by an assortment of geologic factors.
Typically, the sides of a promontory are steep, making it seem more clifflike. The sides may also be deeply ridged, where veins of softer rock have been eaten away to leave the harder stone behind. Over time, the top of a promontory tends to become flat, and deposition creates a layer of soil which may support rugged plants, trees, and grasses. Since a promontory is higher than the surrounding land, it may be extremely windy, which can be rough on flora and fauna.
Many historic fortresses and castles have been built on promontories, since they provide a sweeping view of neighboring lands. Historic promontory forts are widely distributed across Britain and Ireland, since the landscape and weather conditions there promote the formation of promontories. You may also have noticed that the cores of some cities are located on promontories, although urban sprawl may have obscured the shape of the promontory.
On the ocean, a promontory is usually distinguished from features like headlands and peninsulas by its shape. Promontories on the sea are long and narrow, like fingers of land, rather than more broad, like headlands. They are not large enough to be considered true peninsulas, although they share steep cliffs with all of these geologic features. Over time, the link between a promontory and the land will be eaten away, creating an island. This process will also ultimately make islands out of headlands and peninsulas as well, although it will take significantly more time.