On the Martian highlands there are many channels and valleys that have similar characteristics as the dendritic (or "treelike" pattern of branches and twigs seen in deciduous trees) drainage systems on Earth. Figures 4.2 and 4.3 represent two images showing drainage systems at two different scales. Individual segments of the channels were no more than 50 km long and 1 km wide. However, the whole system, consisting of many branching tributaries, might reach 1000 km in length.

Channels could also form complex interconnected patterns as shown in Figure 4.4. This area is near a volcano. The complex patterns were probably created by the repeated major flooding caused by the melting of ground ice during eruptions. Crater frequency in some channel floors indicated that these channels were probably formed 3.5 to 4.5 billion years ago. At that time, the atmosphere might have been more dense and the surface temperature might also have been warmer. As a result, liquid water could have existed on the Martian surface in that particular period of time. This is a simple explanation for the formation of dendritic channels. However, the validity of such an explanation has not yet been established. According to this scenario, for example, rain was possible during the first billion years of Martian history. Dendritic channels were formed by surface runoff. It follows then that an ocean might have existed during this time.

For example, unlike the drainage system on Earth, some Martian channels were isolated and others had large areas between branches that were not dissected. These channels could also appear sinuous (Figure 4.5). They could be better explained by groundwater seepage. A close-up view of Figure 4.5 is given in Figure 4.6 . The lack of dissecting terrain next to the channel is obvious but puzzling.