The Archen fossil is actually found in Undella Bay, which is accessible after defeating the Elite Four and becoming champion.
Here's how to find it:
1. Go to Undella Bay: You'll find this area on the eastern coast of the Unova region.
2. Enter the cave: There is a cave on the eastern side of Undella Bay.
3. Explore the cave: The Archen fossil will be lying on the ground inside the cave.
Remember, you'll need to revive the Archen fossil using the Revive machine in any Pokemon Center. This will give you an Archen, which will evolve into Archeops.