Cobblestone formation
Cobblestones can form as rivers wash them away or as they rub against each other. The cobblestones can be caused by historical floods or mudslides (because they are usually in mountainous areas) but the specific cause depends on the geological structure, the geographical environment, and also the conditions of other surrounding mountains. About 100 million years ago, during the Mesozoic era, the Earth's crust moved as magma rose to the surface. The rapidly cooling magma formed granite. Then, thousands of years ago, an invasion of magma caused granite to be crushed at random, which produced a large number of smaller stones. The stones at the water's edge, after a long period of slow erosion and rolling of their own, eventually formed the pebbles of today. Some are rounded by weathering and become pebbles