Possible Clues:

• High pts.
• Everest and Rainier (Abbr.)
• Rockies, e.g.: Abbr.
• They're often volcanic: Abbr.
• 64-Across and others: Abbr.
• McKinley and Logan, et al (Abbr.)
• The Appalachians, e.g.: Abbr.
• Holyoke and Sinai, e.g.: Abbr.
• High peaks: Abbr.
• Whitney et al.: Abbr.