A shale pyrolysis system includes a retort with a first side and a second side. The second side is opposite the first side and the first side and the second side include descending angled surfaces at alternating angles to produce zig-zag motion of shale descending through the retort. Corners of the retort that change direction of the shale are rounded. The system includes steam distributors coupled to the first side and collectors coupled to the second side to produce crossflow of steam and heat across the descending shale from the first side to the second side, and a steam temperature control subsystem coupled to the steam distributors and configured to deliver higher-temperature steam to one or more upper sections of the retort and lower-temperature steam to one or more lower sections of the retort.