The region between Terra Cimmeria and Terra Sirenum features several enigmatic knob fields located in large degraded basins. These knob fields were first mapped on Viking imagery as chaotic terrain. However, they lack a direct link to an outflow channel and also show other differences to the chaotic terrains of the circum-Chryse region. Therefore, the terms knob field or knobby terrain are used despite their official names like for example Atlantis chaos further to the east, or Caralis Chaos presented here. These regions are believed to represent the remnants of the ancient Eridania lake. Spanning over roughly 1.1 million square kilometers and three large basins, lake Eridania may represent the source region of the large Ma’adim Vallis outflow channel further to the north. The Eridania lake is hypothesized to have existed during the Middle to Late Noachian period. Later in time, the large lake divided into smaller isolated lakes and eventually disappeared altogether along with the rest of the water on the planet.

The lower right side of the image shows a part of Caralis Chaos. As spectroscopic observations indicate, the knobby and light-toned material could represent an ancient airfall deposit that was later covered and altered by water. When the lakes dried out, the material broke up and wind erosion set in. Later in time, the Sirenum Fossae faults overprinted the landscape, crosscutting the knob field and the smooth surface on the left side of the image.

Very interesting is the high amount of wrinkle ridges in the region. Wrinkle ridges are common features on volcanic plains. They develop when the lava sheet is compressed and still slightly soft and elastic. The lava sheet buckles and deforms under the compressional stress triggered by the continuous pressure of the newly produced lava.

Another interesting observation can be made on the southern side of the large, flat-floored impact crater in the central portion of the image. Apparently, several flows and small valleys can be spotted here, presumably indicating the possible presence of water in a later episode. The smaller crater to the south shows small gullies on its northern flank, which seem to have eaten away parts of the crater infill at the bottom. Small valleys can also be spotted at closer inspection in many places outside of the crater, draining into the basin. On the upper right side of the image lies a dissected plateau with a smooth surface. Here, the remnants of several, heavily eroded impact craters can be found.