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).

Coastal plain and Piedmont

Explanation:

A fall line refers to an imaginary line between two parallel rivers and usually at the point where rivers fall at roughly the same height.

Fall line is the north-south running boundary where Virginia rivers drop down from resistant metamorphic rock layers onto flat sedimentary rocks. It separates the Coastal Plain and the Piedmont.