Apply:
overflow:hidden;
to site_content_large_div
...or, if you want the div to grow with its contents, and for its container to work properly, then remove the height value applied to site_content_container_div
, and apply a clearfix to the same, so that it's enclosed, floated elements will contribute to its calculated height.
Clearfix solves a common problem with float-based layouts, so it's best to create a separate, reusable class for this fix that you can apply to any element containing floats that need to be entirely visually contained.
This article provides all the necessary code: http://www.yuiblog.com/blog/2010/09/27/clearfix-reloaded-overflowhidden-demystified/