"There is a line that starts in northeastern Washington and then travels approximately along the border of Washington and Idaho and the border of Oregon and Idaho. This line separates two areas that have different values of Strontium 87. Strontium 87 is an isotope that is stable, meaning it is no longer radioactive. Its parent material is Rubidium 87. Rocks east of this line have a much higher concentration of Strontium 87, while rocks west of this line have a low concentration of Strontium 87. Explain why this occurs."