4th+period

Resources: http://upload.wikimedia.org/wikipedia/commons/0/0b/%D0%A0%D1%82%D1%83%D1%82%D0%BD%D0%B8%D0%B9_%D0%BB%D0%B0%D0%B1%D0%BE%D1%80%D0%B0%D1%82%D0%BE%D1%80%D0%BD%D0%B8%D0%B9_%D1%82%D0%B5%D1%80%D0%BC%D0%BE%D0%BC%D0%B5%D1%82%D1%80.jpg

http://www.doityourself.com/stry/waterbalance http://en.wikipedia.org/wiki/File:Laurencia.jpg http://en.wikipedia.org/wiki/File:WisconsinScenery.jpg http://en.wikipedia.org/wiki/File:SpearfishCreek.jpg