Beaches and mountains are the indirect result of the Flood, which was created by God. There were no oceans prior to the Flood. Most of the Floodwater is out in the oceans.
Before the Flood, there were vast subterranean water chambers that existed under the thinner plates of the Earth's crust. During the Flood, these "fountains of the deep" broke open, and this water came to the surface, collapsing the chambers as they emptied, and creating the ocean basins. The rest of the water came from the collapse of the 7th layer of water/ice above the Earth. These two things combined caused everything on the Earth to be buried 2 miles deep in water. The weight of all this water on the Earth's crust caused the plates to shift and collide with each other and mountain ranges were built. As the thicker plates rose up, and the thinner plates sank, water drained rapidly off the surface, creating formations like the Grand Canyon, which used to be soft sediment at that time, and later mineralized into rock.
Beaches are created by these Flood-created oceans washing sediment against the edge of a land mass.

So, although mountains and beaches were not necessarily part of the original creation, they are definitely post-Flood creations.