Menu changes to vertical format on child theme activation

I’m completely stumped. I’ve created a simple child theme for Twenty Eleven; when I activate it the menu changes from the standard horizontal bar to a large vertical menu. I’ve stripped down the child style.css to the bare minimum to test it and the menu is still appearing when the child is activated. When I switch back to TwentyEleven, everything is fine. I’ve made no changes to any of the header.php or page.php files, done two fresh WP installations, and I’m still having the problem. I’ve never encountered this before with any other WP site I’ve built.
https://www.geraldcampbellart.com