This describes the effect in the Prouty figure in much more detail.
I'm not sure what you don't get - the ground doesn't produce the vortex, it comes from the rotor wash reacting with the ground. The rotor vortices exist without the presence of the ground as well.
The behavior of the rotor wake in the vicinity of the ground is challenging to predict. Under in ground effect(IGE) conditions, the wake collides with the ground and causes a significant perturbation to the flow near the blade. Significant interactions between the main rotor wake and the ground have been associated with the formation and passage of the ground vortex in forward flight. When the helicopter encounters a ground vortex, the stability of the the aircraft is degraded.
The 'rotor wake' is a clue to the origin of the vortices.