Complex sentence: The youngest lava flows could be less than 1500 years old, although many mistakenly believe it to be part of the Cascade Range (it is a member of the Cascade Volcanic Belt, but not of the Cascade Range).
Split into simpler sentences: The youngest lava flows could be less than 1500 years old. Although many mistakenly believe it to be part of the Cascade Range (it is a member of the Cascade Volcanoes, but not of the Cascade Range).