In hilly terrain, radiation fog is most common in low areas such as marshes and river valleys. Please explain?

Cooler air tends to sink while warm air tends to rise. In a valley, it's like a basin where cold air gets trapped and sinks to the bottom. Since it's colder at the bottom, the bottom reaches the dew point sooner and condenses the moisture in the air out as fog. So, you end up with fog at the bottom of the depression in the terrain, but not at the top. This happens a lot in canyons, too, where the fog develops at the bottom of the gorge and flows like slowly moving water toward lower ground.