First, draw a diagram to visualize the situation:
The area of the region covered with small rocks can be found by subtracting the area of the circle from the area of the square.
The area A_s of a square with side L is given by:

And the area A_c of a circle with radius r is given by:

Replace r=4ft and L=10ft into the equations to find the area of the circle and the square:

Finally, subtract the area of the circle from the area of the square to find the area of the region covered with rocks:

Therefore, the area of the region covered with rocks is exactly 100-16π square feet, which is approximately equal to 49.7 square feet.