If hydraulic action and abrasion undercut headlands, structures ...

[gonzo233]

If hydraulic action and abrasion undercut headlands, structures known as ___________________ _
  may form. When these join, they form a(n) ___________________ _, which might later collapse to
  give a(n) ___________________ _. Fill in the blank(s) with the appropriate word(s).

Question 2

A structure built perpendicular to shore to enlarge a beach or prevent erosion is a(n)
  ___________________ _. Fill in the blank(s) with the appropriate word(s).

[mfedorka]

Answer to Question 1

sea caves, sea arch, sea stack

Answer to Question 2

groin

[gonzo233]

Great answer, keep it coming

[bimper21]

Gracias!