The Fall line which is the north-south running boundary where Virginia
rivers drop down from resistant metamorphic rock layers onto flat,
sedimentary separates what 2 provinces (regions).

The fall line  which is the north-south running boundary where Virginia  rivers drop down from resistant metamorphic rock layers onto flat,  sedimentary separates:

The Coastal Plain (Tidewater) and Piedmont.

Explanation:

The reason behind this answer is that the fall line is an imaginary line that separates two rivers in basically the core or center of Virginia State. However, it starts in the northeast and crosses all over the state to reach the southwest. it separates two different sections of terrain with different elevations. So, practically it is the natural frontier between two regions called Piedmont, and The Coastal Plain.