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Some areas of the Moon’s surface are crisscrossed by kilometers-long linear depression features; “cracks” as Wilkins and others called them. Many of these features do appear to be simply cracks, but their true nature is more complex than that. The classification of these linear features depends primarily on their particular creative processes. The shallow fault trace or meandering river-like features are all called rilles. The generally deeper and wider linear depressions are known as either a catena (crater chain) or a vallis (valley). In this chapter we will observe these linear features and cover the different endogenic and external processes by which they were formed.

Whether they are straight, arcuate, or sinuous, rimae (rilles) appear to be channels, which may be the result of faulting, lava flows, collapsed lava tubes, or extensional tectonic processes such as occurrences when basin-forming impacts were later filled with dense basaltic lavas. Straight rilles are frequently grabens, a collapsed floor between two parallel faults that can extend for hundreds of kilometers. Arcuate rilles are often found around the margins of maria when the stress of the cooling mass of basalt lava faulted the crust in concentric arcs. Sinuous rilles resulted when flowing lava formed channels or tubes that, when emptied or collapsed, revealed the snaking path of the lava caused by the differential velocity of the flow.