a road that is raised above water or marshland or sand
"The old ferry service was replaced by a sturdy causeway that cuts through the muddy tidal flats to connect the two islands."
A road that is raised, so as to be above water, marshland, and similar low-lying obstacles. Originally causeways were much like dykes, generally pierced to let water through, whereas many modern causeways are more like bridges or viaducts.
"The old ferry service was replaced by a sturdy concrete causeway that allowed cars to drive directly over the marsh without getting wet."
pave a road with cobblestones or pebbles
"The old village causeway was paved with smooth river stones that sparkled under the morning sun."
To pave, to cobble.
"The villagers gathered stones to causeway the muddy track leading to the harvest fields."