Posts getting stuck within posts?

So I’ve been modifying the sight theme and ran up against a problem. In my content area, for some reason my second dummy post is getting pushed right; it seems like it’s stuck within the first/more recent dummy post. Consequently, my sidebar is awkwardly getting pushed down to the bottom of the page.

Assuming it has to do something with padding, but I can’t figure out what.

uscfractal.com