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That formation is called an arch, and it connects what may soon become a stack to the main coastline. The key points of both archs and stacks is that they form by erosion. In contrast, a peninsula...
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Source: https://writers.stackexchange.com/a/47058 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
That formation is called an [arch](https://en.wikipedia.org/wiki/Natural_arch), and it connects what may soon become a [stack](https://en.wikipedia.org/wiki/Stack_(geology)) to the main coastline. The key points of both archs and stacks is that they form by erosion. In contrast, a peninsula is **typically** a larger landmass formed by accretion or alternative means other than erosion. Italy is a peninsula, to give an example. The links: 1. [https://en.wikipedia.org/wiki/Natural\_arch](https://en.wikipedia.org/wiki/Natural_arch) 2. [https://en.wikipedia.org/wiki/Stack\_(geology)](https://en.wikipedia.org/wiki/Stack_(geology))