Query SiteTree for archived pages

I have several pages in my site that are in an archived state but cannot see what Silverstripe uses to determine that a page is in an archived state.

Is there a function that I can use to determine whether a page is in an archived state, something like

foreach (SiteTree::get() as $page) {
    echo $page->isArchived();
}

Or some sort of attribute that I can query by to return all archived pages?