Simplify the following text for a Grade 10 native English-speaking person.
Original: The Flint Hills , historically known as Bluestem Pastures or Blue Stem Hills , are a band of hills in eastern Kansas stretching into north-central Oklahoma , extending from Marshall County in the north to Cowley County , Kansas and Osage County , Oklahoma in the south .
Simplified: The Flint Hills , historically known as Bluestem Pastures , are a band of hills in eastern Kansas stretching into north-central Oklahoma .