Need help fixing responsive CSS so mobile navigation menu pushes down content instead of overlapping it

I'm currently working on the following responsive html5 homepage template for a Wordpress site:

http://wordpress.silvermanacampora.com/wp-content/themes/silvermanacampora/index-new.html

The problem I'm having is -- when viewing the site on my iPhone -- and when pressing the "Main Menu" button (which toggles the displaying of a very tall navigation menu) --- the navigation menu is overlapping and hiding all of the content that's underneath it -- instead of pushing it down.  

I suspect that there may be a problem with my having specified a height value to the navigation menu:

ul#hp_navigation { float:none; display:block; width:100%; background-color:#00334E !important; text-align:center; margin:0 0 0 0; border-top:1px solid #fff; height:825px; }	

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However -- if I don't specify a height value -- then the Main Menu button doesn't move down to underneath the expanded menu like it's supposed to.

What do I need to do / change in my CSS in order to fix this problem?

Thanks,
- Yvan

P.S. I have all of this working correctly elsewhere on the Wordpress site (ie: http://wordpress.silvermanacampora.com/who/ ) -- but I can't seem to figure out what's different about my new homepage that's resulting in this odd behavior.