By focusing a femtosecond laser beam in the bulk of a transparent material one can locally change the physical properties of the material itself. The interaction between NIR pulses and fused silica has been extensively studied in the past, showing how various types of modifications can be induced. In particular, type I (waveguides) [1] and type II (nanogratings) [2] modifications allow to respectively inscribe waveguiding tracks and locally change the material etching rate. Lately, it was shown that UV femtosecond lasers can be used to inscribe waveguides-like modification at a higher speed than NIR-type I and to machine etchable structures with a higher etching rate than NIR-type II [3]. Here, we further study the interaction between UV femtosecond pulses and fused silica and report a novel regime where self-organised highly periodic structures are present.