Origami folds and bases

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Across
  1. 7. before a more important event
  2. 9. between hills and mountains
  3. 10. flexible bag of air
  4. 13. to corrugate
  5. 15. kilts have these
  6. 18. parrot, sparrow, flamingo
  7. 19. a rhombus flying high
  8. 20. point of entry
  9. 21. green outer whorl of a flower
Down
  1. 1. elevated portion of the earth’s crust
  2. 2. set of steps
  3. 3. H2O explosive
  4. 4. apple, oranges and pears
  5. 5. modified leaves (corolla)
  6. 6. gill-bearing animals
  7. 8. backwards
  8. 11. numbers arranged in a rectangle used as a single unit
  9. 12. an amphibian
  10. 14. small bag attached to clothing or luggage
  11. 16. last letter of the alphabet
  12. 17. to turn on a central point