Gravel originates from the cyclical deposition of iron oxide in the mottled layer of the soil.
When the soil is wet, the ferrous iron in the soil moves up the profile by diffusion, as atmospheric oxygen simultaneously diffuses down through the water. A chemical reaction occurs where these meet, producing insoluble ferric iron. As this process proceeds over several cycles, deposition of the ferric iron continues around a particle. This forms concentric layers and causes it to be extremely rounded. When the mottled zone weathers these particles remain as gravel.
Due to the extreme roundness of the gravel pieces, walking on the stuff is actually fairly slippery and it easy to loose your footing, especially walking up or down a slope.
In Australia, gravel is used as surface for unsealed roads, particularly found in country areas.