Simplify the following text for a Grade 6 native English-speaking person.
Original: The Wrangell Mountains are almost entirely volcanic in origin , and they include the second and third highest volcanoes in the United States , Mount Blackburn and Mount Sanford .
Simplified: The Wrangell Mountains were made by volcanoes . The second and third tallest volcanoes in the United States , Mount Blackburn and Mount Sanford are there .