Every (navigation and) subnavigation folder requires an index page. To create one, click on New, Page, Navigation Index Page:
Leave the System Name as 'index'. Give the page a title (eg. Eighteenth Century History) - this is the text that appears in the navigation, indicate whether you want this item to show in the top navigation and/or in the left navigation on landing (or secondary content pages). If you wish to have a different page title than indicated in the Title field, enter it in the Alternative Page Title field. Add the content for this page now, or you can choose to do so later. Click Submit:
You will see the index file associated with the Subfolder in your folder list:
Published on and maintained in Cascade CMS.