Complex sentence: Thielsen is part of the High Cascades, a branch of the main Cascades range that includes Oregonian volcanoes less than 3.5 million years old; it is also a member of a group of extinct volcanoes distinguished by their sharp peaks.
Split into simpler sentences: Thielsen is part of the High Cascades, a branch of the main Cascades range that includes Oregonian volcanoes less than 3.5 million years old. It is a member of a group of extinct volcanoes distinguished by their sharp peaks.