functions.php

<?php

/**
 * Section Menu
 * Displays the subpages of the current section
 *
 * @author Bill Erickson
 * @link http://www.billerickson.net/custom-secondary-menu
 */
function be_section_menu() {

	// Only run on pages
	if( !is_page() )
		return;

	// If top level page, use current ID; else use highest ancestor
	global $post;
	$section_id = empty( $post->ancestors ) ? $post->ID : end( $post->ancestors );

	// Get all the menu locations
	$locations = get_nav_menu_locations();
	// Find out which menu is in the 'primary' location
	$menu = wp_get_nav_menu_object( $locations[ 'primary' ] );
	// Grab all menu items in this menu that have a parent of the current section.
	// This grabs the subpages, assuming the current section is a top level page
	$menu_items = wp_get_nav_menu_items( $menu->term_id, array( 'post_parent' => $section_id ) );
	// If there are menu items, build the menu
	if( !empty( $menu_items ) ) {
		echo '<ul class="section-submenu">';
		$first = true;
		foreach( $menu_items as $menu_item ) {
			$classes = 'page-item';
			// This adds a class to the first item so I can style it differently
			if( $first )
				$classes .= ' first-menu-item';
			$first = false;
			// This marks the current menu item
			if( get_the_ID() == $menu_item->object_id )
				$classes .= ' current_page_item';
			echo '<li class="' . $classes . '"><a href="' . $menu_item->url . '">' . $menu_item->title . '</a></li>';
		}
		echo '</ul>';
	}

}
add_action( 'genesis_before_loop', 'be_section_menu' );