It looks like the road is built with rocks, then finer rocks, and gravel, and then finer gravel, and so on gradually building up until there is a smooth and solid base that can be paved.
Pan-American Highway Construction
Pan-American Highway Construction
It looks like the road is built with rocks, then finer rocks, and gravel, and then finer gravel, and so on gradually building up until there is a smooth and solid base that can be paved.
The Panama Adventure 
