Sidebar overlays content in mobile grid

I've created a subtheme of omega 7.x-3.0-rc5. The responsive grid performs as expected in the wide, normal and narrow modes. When I reduce my browser window to mobile size the sidebar content overlays some of the main content. I thought the sidebars are supposed to stack below the main content when the browser window is reduced below say 420px. I've attached 2 screenshots to illustrate: 1. the narrow grid (narrowgrid.GIF) 2. the mobile grid (defaultgrid.GIF). Hopefully I've just missed some configuration setting rather than having discovered a bug.

I'm on Drupal 7.7 with omega 7.x-3.0-rc5