Abstract
The earthen border wall (Great Wall) built by the Ming is largely made of wind-blown loess. However, does the composition of this loess change along the length of the wall in response to variations in regional sediment transport pathways and impacting on the wall’s erosional durability? To date, defining these sediment transport pathways has been a challenge because of the paucity of spatially-comprehensive, compositional information. Here, we show that satellite ASTER mineral maps, combined with field sample measurements along a 1200 km section of the Ming’s earthen wall, reveal both the compositional heterogeneity of loess as well as the complexity of the sediment transport pathways of individual loess components, including: (i) quartz sand from Cretaceous sandstones in the Gobi Desert; (ii) gypsum from evaporative lakes in the Tengger Desert; (iii) kaolinite from Devonian Molasse in the Qilian Shan; and (iv) chlorite and muscovite from meta-volcanic rocks exposed across the Alashan Block. Sediment transport pathways involve a combination of colluvial, aeolian and fluvial (ephemeral and permanent) processes shaped by the topography. ASTER enabled mapping of compositional gradients related to two pathways, namely: (i) quartz sand driven by aeolian saltation in concert with the Yellow River; and (ii) clay and fine silt travelling large distances (>500 km) by long-term wind suspension. The most intact section of wall is found along the Hexi Corridor, which is poor in quartz sand and rich in (kaolinitic) clay and fine-silt, driven by wind-shielding by the Alashan Block. We also found evidence that the Ming: (i) mined loess from close by the wall (<1 km); (ii) targeted loess richer in finer fractions; and (iii) routinely applied a Ca-rich additive (probably lime).
Highlights
The Ming Dynasty’s program of building an earthen border wall began following a decisive battle that forced the Ordos Mongols north of the Yellow River in 1473 [1]
The composition of loess used by the Ming to build their earthen wall across the Ordos Plateau and Hexi Corridor is heterogeneous
The ASTER mineral maps enable the tracking of sediment transport pathways of loess related minerals not detected in previous studies relying on point-sample data
Summary
The Ming Dynasty’s program of building an earthen border wall began following a decisive battle that forced the Ordos Mongols north of the Yellow River in 1473 [1]. The Ming were able to build this wall to a height of 5–8 m (Figure 1d,e) in just three months using their “hangtu” method of ramming earth between formwork [1,3]. This technique resulted in a characteristic 15–30 cm horizontal layering [4] and contrasts with that built by earlier dynasties where a thinner (7–10 cm) layering of earth was often. MEvineng stheloeucgtehdwloinedss-gfeonrewraatleldcolonesstsru(cdtoiomninbaetceadusbeyoafnitgsuplaerr,vsaislti-vseizde egvrealionps)ment at/neadrepthoesitssuarrfearceeadaiclryoesrsodthede [O6,r7d],otshePMlaitnegasue,leecstpedecloiaelslsyfosrowutahll ocofntshtreuMctiuonUbsecDauesseerotf i(tFsipgeurrveassi1vae and
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