Question

How are stacks, caves, blowholes, and cliffs formed


Just judging by their names, I believe water erosion causes these. You can check though -- try these:

http://www.answers.com/blowholes
and
http://www.google.com

Search for <b>stacks formation</b> or <b>caves formation</b>, etc.

=)

Check these sites for brief explanations and diagrams of the formations of these features.

http://www.geography.learnontheinternet.co.uk/topics/waves.html

http://revisioncentre.co.uk/gcse/geography/coastal_formations.html