The Medusae Fossae Formation is a soft, easily eroded deposit that extends (discontinuously) for more than 5,000 km along the equator of Mars. It has an area equal to 20% the size of the continental United States. Sometimes, the formation appears as a smooth and gently undulating surface, however in places it is wind-sculpted into ridges and grooves. Radar imaging has suggested that the region may contain either extremely porous rock (for example volcanic ash) or deep layers of glacier-like ice deposits amounting to about the same quantity as is stored in Mars' south polar cap. Using a global climate model, a group of researchers headed by Laura Kerber found that the Medusae Fossae Formation could have been formed from ash from the volcanoes Apollinaris Mons, Arsia Mons, and possibly Pavonis Mons. Further evidence for a fine-grained composition is the fact that the area gives almost no radar return. For this reason it has been called a stealth region. The formation is divided into three subunits (members) that are all considered to be of Amazonian age, the youngest era in martian geological history. Comparisons of elemental composition suggest that the Medusae Fossae Formation has been the main source of Mars' ubiquitous surface dust. In July 2018, researchers reported that, indeed, the largest single source of dust on the planet Mars is the Medusae Fossae Formation.