Gap appearing between menu and content

Attached is a screenshot of a gap that appears when adding a block to any of the preface regions.
Adding blocks to regions Hero 1 or Hero 2 do not cause this. Using Firebug I haven't been able to track this down.
This also occurs when adding content to any preface region with content already in regions Hero 1 and Hero 2 which is a little strange since preface is lower on the page. I thought this might be due to some customization I was doing but the screenshot is from a clean install of D-7.20 with Bamboo alpha15.