Help with a get_categories () drop down menu – I want it to show in heirachial form

wp_dropdown_categories has a hierarchical and depth options available.

$args = array(
    'show_option_all'    => 'All Tshirt Categories',
    'orderby'            => 'ID', 
    'order'              => 'ASC',
    'hide_empty'         => 1, 
    'child_of'           => 0,
    'hierarchical'       => 1, 
    'depth'              => 1,
    'taxonomy'           => 'tshirt_categories',
    'hide_if_empty'      => false ); ?>


<h2>By Category</h2>
    <form action="<?php bloginfo('url'); ?>" method="get">
    <div>
    <?php wp_dropdown_categories($args); ?>
    <input type="submit" name="submit" value="view" />
    </div>
    </form>