Fill dirt is earthy material used to fill in holes or depression in the ground or create dirt mounds that would change the elevation or grade of an existing land area. If you spend some time digging underneath the top layer of the earth, you’ll certainly find plenty of fill dirt. Because fill dirt doesn’t contain any organic matter, only broken down clay, sand, and rocks, it’s much more stable than other materials. Fill dirt won’t shift or move and is used as a perfect filler for depressions in your property.