This was a tricky one, because the word "erg" has more than one meaning. In physics, it is a unit of measure for mechanical work, but in geology, it means a large area that is all sand. For example, the Sahara Desert is made up of several ergs, or sand seas. The cliffs behind my house are Navajo Sandstone that formed from one of the largest ergs in our planet's history. During the Jurassic period (about 160 million years ago), it covered large parts of Utah, Arizona, New Mexico, Colorado, and Wyoming. In some places, this sandstone is up to 2,500 feet thick. The dunes formed the angled layers, called crossbedding, and in some places it contains dinosaur footprints on the dune faces. I have not found any dinosaur fossils in our cliff, but I am still looking.