Find the period -- from cross section??

Here is a picture (looking down from above) of striped sand ripples from Sea Rim State Park in E. Texas. The bottom of the picture is south and the top is north. The ripple pattern is 24 inches from south to north. Think about taking a cross section that cuts from the bottom of the picture to the top. The height of the sand along this cross section is somewhat periodic. Imagine the period function p(y) describing the height of the sand as a function of the distance in inches north from the southern edge of the pictured pattern.

__WHAT IS THE PERIOD???__

http://dl.dropbox.com/u/195969/IMG_6368.jpg

Thanks so much for any help. I think I'm just confused by the wording.