Islands are named that way because their appearance looks like an island – a time zone infraction in which the middle sticks out above the others. Another term for this pattern is the head-and-shoulders pattern.
Islands are the most ambiguous scenarios because while they indicate that a problem existed in the past, it is more difficult to ascertain that condition now. A drop in outbound spam from the previous hour could mean two things:
Valleys are so-named because the visual pattern they form looks like a valley; the current run and the run from two hours ago are greater in size than the one from one hour ago.
Valleys, similar to mountains, generally give very unambiguous signals that a spam problem has occurred. It is usually a very clear signal that a sudden shift in outbound spam has occurred because the current run so greatly dwarfs the previous one.