...created sinuous rilles and large fissures.
Sinuous rilles (see annotated image) are another common surface feature found at the flanks of volcanos. They are commonly smaller than the pit structures and describe rimless, sinuous channels. Volcanic processes such as ash flow erosion or lava flow emplacement, but also surface water flow and a combination of volcanic and water activity have been suggested as formation mechanisms for sinuous rilles.
The left side of the image is dominated by several large fissures, reaching length of up to 40 km. Braided channel networks emerge from these fissures. Along the channels, streamlined islands and terraces in the channel walls can be spotted (see annotated image). This suggests a formation by water and not by lava. Presumably snow/ice deposits accumulated on the flanks of the volcano and were buried under subsequent volcanic airfall deposits. They were then melted in a later active episode of Ascraeus Mons.