We present high spatial resolution (750 au at 250 pc) maps of the B1 shock in the blue lobe of the L1157 outflow in four lines: CS (3-2), CH3OH (3K-2K), HC3N (16-15) and p-H2CO (202-301). The combined analysis of the morphology and spectral profiles has shown that the highest velocity gas is confined in a few compact ( 5 arcsec) bullets, while the lowest velocity gas traces the wall of the gas cavity excavated by the shock expansion. A large velocity gradient model applied to the CS (3-2) and (2-1) lines provides an upper limit of 106 cm−3 to the averaged gas density in B1 and a range of  cm−3 for the density of the high-velocity bullets. The origin of the bullets is still uncertain: they could be the result of local instabilities produced by the interaction of the jet with the ambient medium or could be clumps already present in the ambient medium that are excited and accelerated by the expanding outflows. The column densities of the observed species can be reproduced qualitatively by the presence in B1 of a C-type shock and only models where the gas reaches temperatures of at least 4000 K can reproduce the observed HC3N column density.