Continuing down Hwy 385 in the Marathon Basin, one of the most spectacular sights are the flatirons on East Bourland Mountain, photographed 8 miles south of Hwy 90. A flatiron is a short, triangular hogback forming a ridge or spur on the flank of a hill that looks like a flatiron. A flatiron is usually a plate of steeply inclined resistant rock, in this case Caballos Novaculite, called a flatiron from its shape. A flatiron is a short, triangular hogback forming a ridge or spur on the flank of a hill that looks like a flatiron. Here, the light-colored flatirons of novaculite stand out against dark Maravillas Chert on the higher parts of the mountain. The Maravillas Chert Formation is of bedded black chert and dark gray to black limestone, 100 to 400 feet thick, the next youngest to the novaculite in the Marathon Basin succession.