But  how exactly does deposition occur?

Sediments are often deposited by water, and different environments deposit different types of sediments.  Energetic environments, such as near the shoreline or in a fast moving river, move and deposit larger grains.  Less energetic environments, such as in the deep ocean, move and deposit smaller grains. 

As the sea moves in and out over time, the water depth and depositional environments at any given location change over time, causing the type of sediment deposited at that location to change with time. 

The deposition occurring in the movie in the center panel represents the red spot on the map on the left.  During a transgression that occurred in Cambrian times, note how the sediment type changes as the depth of water changes.  The photograph on the right shows an example of the landscape produced by this transgressive event and subsequent erosion.

part 3, pg 22