Possible Clues:

• Land between hills
• San Fernando, for one
• Area between mountains
• ___ girl (classic ditzy type)
• See friend swallowing drug for depression
• Geological depression, and what the first word of 20-, 37- and 56-Across is
• TVA center
• A little way down here after five
• Where, with volume, a yodel comes back?
• Dale, dene or glen