Before revision: For example, in a detrital record , there are zircon grains with the age of 1.0 billion years , but there are two source areas upstream that yield 1.0 billion years old zircon and rivers drained through both area.
After revision: For example, if a detrital record contains zircon grains with an age of one billion years that were transported by rivers flowing through two source areas containing zircons which are also one billion years old zircon and rivers drained through both area.