Creating a dropdown list item menu with CSS only – Photoshop

In a section of website I’m working on I have a NAV element that contains three sections: About, Portfolio, Contact. I’m trying to make it so that when you hover over the Portfolio section, a drop down appears allowing you to choose between two other sections, “Writing Samples” and “Photoshop.” I am trying to accomplish this using only CSS.

This is my HTML section:

<nav>
            <ul>
                <li>
                    <a href="index.html" >About</a>
                </li>
                <li class="subNav">
                    <a class="selected" >Portfolio</a>
                    <ul>
                        <li><a href="writing_samples.html">Writing Samples</a></li>
                        <li><a href="photoshop.html">Photoshop</a></li>
                    </ul>
                </li>
                <li>
                    <a href="contact.html">Contact</a>
                </li>
            </ul>
        </nav>

And CSS:

nav {
position: absolute;
bottom: 0;
right: 0;
padding: 10px 0;
} 

nav ul {
list-style: none;
margin: 0 10px;
padding: 0;
}

nav li {
display: inline-block;
}

nav a {
font-weight: 800;
padding: 15px 10px;
}

nav ul li.subNav ul {
display: none;
} 

nav ul li.subNav:hover ul {
display: block;
}

I have reached the point that when I hover over the Portfolio list item, you see the resulting list items “Writing Samples” and “Photoshop”, except that it displays these two items as a part of the original unordered list, and moves the “Portfolio” list item above the rest of the items. I would like “Writing Samples” and “Photoshop” to appear vertically under “Portfolio”, but I can’t quite figure this out with CSS.