a, Schematic plan of the subsections of palaeomagnetic sampling. The numbers 800 and 900 refer to the height in metres above sea level. The contour interval is 20 m. Subsection KW refers to the additional parallel section from L25 to L28 with the excavation into S27 and L28. b, Schematic of the method used to link the five subsections in the Shangchen locality. This figure and Extended Data Fig. 4 show the loess–palaeosol stratigraphy, with key marker layers, sampled subsections and the locations where artefacts were found. The details of the method used to link the five subsections are given below. Subsections I and II are linked on the basis of the L15/S15 boundary. Measured horizons of subsection I start from the mid-to-lower part of L5 and run downward to the base of marker layer L15 (to the boundary of L15/S15). Measured horizons of subsection II start from the lowest part of L15 and run downward to the lower part of L25. Because the measured thickness of marker layer L15 is the same in both subsections I and II, we cut out the overlapped L15 in subsection II. Thus, subsection I ends at the base of L15, and subsection II begins at the top of S15. Subsections II and III are linked on the basis of the top boundary of the Olduvai subchron. A small normal-polarity segment of the Olduvai subchron was detected in the marker layer L25 at the base of II. Measured horizons of subsection III start from the lower part of L25 to the top of L27, and record the entire Olduvai subchron with a small reversed-polarity segment on the top (in L25) and at the base (top of L27) of the subsection. Thus, the overlapped normal-polarity segment of the Olduvai subchron at the base of subsection II and a small reverse-polarity segment on top of subsection III were cut out. The reverse-polarity segment at the base of subsection II is linked to the normal-polarity top segment of subsection III, and the whole normal-polarity segment of the Olduvai subchron is contained within subsection III. Subsections III and IV were linked on the basis of the base of the Olduvai subchron. Measured horizons of subsection IV start from the top of L27 to the mid-to-upper part of L28, with a small normal-polarity segment of the base of the Olduvai subchron at the top. After cutting out both the small reverse-polarity segment at the base of subsection III and the small normal-polarity segment on top of subsection IV, the reverse-polarity segment just below the Olduvai subchron in subsection IV is linked to the normal-polarity base of the Olduvai segment in subsection III. Subsection KW contains horizons of L25 (marker layer) to L28, including units L25, S25, L26, S26, L27, S27 and L28. Against the general background of reversed magnetostratigraphy, two normal magnetozones—the Olduvai subchron (1.78–1.95 Ma) and the Réunion excursion (2.13–2.15 Ma)—are recorded. Therefore, section IV and section KW are linked by the base of S27 (that is, S27 at the base of section IV is linked to L28 at the top of subsection KW). Subsection V is opposite of subsection II, which is on the other side of a narrow gully and has two distinct marker layers, L15 in the upper half and L25 in the lower half. There is a palaeosol layer with many artefacts in the middle part of subsection V. The stratigraphic correlation and horizon number demonstrate that this layer belongs to S22. Palaeomagnetic measurements on samples collected from this parallel section all showed reversed polarity.