• wrinkling
• plasticity
• pattern formation
• instability

This work identifies a new pattern formation mechanism, which takes place when compressive forces are exerted on an amorphous solid film having a low elastic limit. Pattern formation in compressed solid films is a very active research field attracting significant experimental and theoretical efforts. To the best of our knowledge, the idea that surface patterns may emerge solely due to the plastic response of the film is simply absent from the literature. As a consequence, the role of plasticity in pattern formation may have been missed in various experimental studies. It is also noteworthy that a few experimental studies did recognize a possible role of plasticity but, for lack of any theory, struggled to explain its mechanisms of action, and never envisioned that it could actually be a primary driver of pattern formation. It thus seems important today to formulate a theoretical framework to understand its underlying physical mechanisms. We do so here by considering an idealized model of a glassy film strongly adhering to a perfectly rigid and smooth substrate and subjected to uniaxial compression. We show that the film response is plastic and leads to the emergence of a series of ridges perpendicular to the compression axis. Then, we precisely identify the fundamental physical mechanisms at their origin and thus show that this phenomenon results from well-known features of the plastic response of glassy materials.