and is covered with fractured plates and sinuous ridges that look like pack-ice on the Earth. Sorted patterned ground and erosion patterns in polygonal terrain in the region support ice-rich material; hence a lake. Also, the presence of streamlined islands, cataracts, and dendritic channel systems suggest formation by water from a lake. Some surfaces here show Rootless cones which are mounds with pits. They can be caused by explosions of lava with ground ice when lava flows on top of ice-rich ground. The ice melts and turns into a vapor that expands in an explosion that produces a cone or ring. Features like these are found in Iceland, when lavas cover water-saturated substrates. The western Elysium Planitia basin can be described as almost a perfect equipotential surface because it slopes only about 10 m over a 500 km distance—that's about as level as the Earth's ocean. This very gentle slope argues against a lava flow. In places, it has been found that the flow surface has been lowered by 50% which is expected if the flow was of water, but not if it were lava. The maximum depth of the lake was estimated to be between 31 and 53 m.