Natural Arches are formed by Erosion and Weathering to cliffs.
If an arch has formed by the sea, it has been formed by weathering and erosion, but if it was formed by a river, it has been formed the erosion from the river.
This is how most seaside arches are formed. Firstly, eater will get squashed into cracks by waves, causing vertical joints to appear. Then, a small cave will appear as the joints get larger and larger. As the sea wears away at it the cave will go through the cliff, causing a hole. This is called an arch.
There are very few river archs in Britain.