The flat topped fog in a valley is actually quite common, the flat top delineating the boundary where the temperature is on the dew point. I saw it often when I lived in California, we were high enough that many mornings I could climb the hill behind the house and see the blanket of fog over most of the valley, with the high spots becoming "islands" sticking up. I could see it sometimes late at night, but cursed it because I was usually driving home from work, and fog isn't what I was looking for.