Southern blue-shifted lobe L1157 in CO (1–0) (white contours; Gueth et al. 1996). The precessing jet ejected by the central object L1157-mm (white star) excavated two cavities, with apices B1 and B2, respectively. The maps are centred at , δ(J2000) = +68°01′105 (Δα = +25″ and Δδ = –635 from L1157-mm). The emission map of the NH₂CHO (4_1,4–3_1,3) line (integrated over the velocity range –5 to +5 km s^-1) is shown by the colour image. For the CO image, the first contour and step are 6σ (1σ = 0.5 Jy beam^-1 km s^-1) and 4σ, respectively. The first contour and step of the NH₂CHO map correspond to 3σ (15 mJy beam^-1 km s^-1) and 1σ, respectively. The dashed circle indicates the primary beam of the NH₂CHO image (64″). The magenta and white ellipses depict the synthesised beams of the NH₂CHO (579 × 481, PA = –94°) and CO (365 × 296, PA = +88°) observations, respectively. The three dashed arrows indicate the directions (projected on the plane of the sky) of the episodic jet producing the shocks analysed in Sect. 3 and Fig. 2.