Sandbox theme Main Menu problem

The following function appears to regulate the main menu in the Sandbox theme and causes the unordered list to display horizontally. I would like to revert to a standard list (displaying vertically), on a site, but every time I adjust the code I break everything:

// Produces a list of pages in the header without whitespace
function sandbox_globalnav() {
	if ( $menu = str_replace( array( "\r", "\n", "\t" ), '', wp_list_pages('title_li=&sort_column=menu_order&echo=0') ) )
		$menu = '<ul>' . $menu . '</ul>';
	$menu = '<div id="menu">' . $menu . "</div>\n";
	echo apply_filters( 'globalnav_menu', $menu ); // Filter to override default globalnav: globalnav_menu
}