Simplify the following text for a Grade 15 native English-speaking person.
Original: The Ordovician , named after the Welsh tribe of the Ordovices , was defined by Charles Lapworth in 1879 , to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison , who were placing the same rock beds in northern Wales into the Cambrian and Silurian periods respectively .
Simplified: The Ordovician , named after the Celtic tribe of the Ordovices , was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison , who were placing the same rock beds in northern Wales into the Cambrian and Silurian periods respectively .