What physical features which lies between two large bodies of water is the northern border to the Muslim Empire

The physical features are refered to the Caucasus mountains, where you can find the Mount Elbrus, considered the highest mountain in Europe (15.554 ft).

Explanation:

The two large bodies of water were the Caspian Sea and the Black sea. The northern border to the Muslim Empire is nowadays known as Georgia.

Umayyad Caliphate spread east into Persia and west across North Africa Arab Muslims were put in the Capital of Arab Empire during Umayyad Dynasty.