Unveiling Millennia: A Geological Journey into Hulu Cave, Tangshan – A High-Resolution Record of East Asian Monsoon and Anthropological Evolution

In May 2025, a group of international researchers, students, and faculty embarked on a geological field excursion to the legendary Hulu Cave, also known as the Nanjing Homo erectus Cave, located in the eastern part of the Tangshan Hills near Nanjing, China. This field trip was not only a rare opportunity to explore a pivotal paleoenvironmental archive physically but also to deepen our understanding of past climate variability, human evolution, and karst geomorphology.

The Hulu Cave: A Natural Laboratory

Situated at 32°3′N, 119°2′E, Hulu Cave is carved into the north slope of Leigongshan Hill, with an entrance that opens northward, 21 meters above the cave floor. The cave spans 64 meters in length, 25 meters in width, and has a height difference of about 25 meters between its roof and floor. Its internal structure is strongly influenced by the underlying limestone formations and associated tectonic structures, giving it a unique morphology characterized by streamlined erosion planes along its walls.

Our group is at the entrance area of the Tangshan Geopark, marking the beginning of our exploration of the Hulu Cave system.

Researchers and students gather inside the Hulu Cave to examine its stratigraphy and geomorphology. The dramatic lighting showcases the cave’s limestone features and karst textures.

The Formation and Evolution of Hulu Cave

The geological history of Hulu Cave can be understood in three distinct stages:

1. Cave Formation and Deep Burial Stage (Late Neogene - Early Pleistocene): During a period of tectonic stability, the ancient topography was subject to erosion and leveling. A stable groundwater level in this environment was conducive to the dissolution of limestone, leading to the initial formation and subsequent enlargement of karst cavities like Hulu Cave. This foundational stage set the scene for the cave's later roles as a repository of sediments and fossils.

The vast chamber of Hulu Cave, showcasing the layered limestone strata and the effects of ancient water flow that sculpted the cave's interior. The lighting accentuates the textures and colors of the rock.

A stunning view of the cave's interior, with illuminated pathways guiding visitors through chambers adorned with stalactites and other speleothems. The scale of the cave is truly impressive.

2. Cave Filling Stage (Post-Early Pleistocene):

Following the Early Pleistocene, the cave began to accumulate deposits. Initially, silty clay and clay beds with distinct bedding planes were laid down, suggesting a stagnant water environment – a "wet" filling stage. Regional neotectonic movements subsequently led to the dissection and erosion of the surrounding hills. This, in turn, caused a lowering of the groundwater level, leaving the cave dry. During this drier phase, unbedded deposits accumulated. As fissures in the limestone dissolved and enlarged, sheet flows transported weathered materials from the slope, along with animal bones and, significantly, the skulls of Homo erectus, into the cave.

Close-up of flowstone formations within Hulu Cave. The intricate layers and textures are a testament to millennia of mineral deposition by seeping water.

A magnificent curtain or shield formation, showcasing the beauty and variety of speleothems found within Hulu Cave. The lighting highlights its delicate structure.

Impressive stalactites and stalagmites demonstrate the slow but persistent process of calcite deposition that forms these iconic cave features.

Fossilized bone remains discovered within the cave sediments, offering direct evidence of the fauna (and potentially hominins) that once inhabited or were washed into Hulu Cave.

3. Accumulation of the Cone-Shaped Deposit and Cave Closure (Later Pleistocene):

Prolonged weathering and denudation caused the hill slope to retreat, which consequently enlarged the cave entrance. This facilitated the accumulation of a large cone of detritus, composed of weathered material, abundant large limestone rubbles, and some mammal skeletons. Eventually, this accumulation of rubble effectively blocked the original cave entrance, sealing its contents for millennia.

Dr. Aadil is standing near a prominent speleothem formation within Hulu Cave, providing a sense of scale to these natural sculptures.

A Window into the Past: Homo erectus and Monsoon Records

The discovery of two Homo erectus skulls within Hulu Cave has cemented its place as a key paleoanthropological site. These remains provide invaluable insights into early human presence and evolution in the region.

Furthermore, the speleothems (cave formations like stalagmites) within Hulu Cave have proven to be exceptional archives of past climate. Studies on these formations, particularly stalagmites, have yielded high-resolution, absolute-dated records of the East Asian Monsoon, stretching back into the Late Pleistocene. These records allow scientists to reconstruct past rainfall patterns and understand the dynamics of this critical climate system with remarkable precision. The work by Wang et al. (2001) on a Late Pleistocene monsoon record and Cheng et al. (2018) on atmospheric radiocarbon changes are landmark studies underscoring the cave's climatic significance.

Our visit to Hulu Cave was a profound reminder of the dynamic interplay between geological processes, climate change, and the story of life on Earth, including our ancient ancestors. The ongoing research at this site continues to shed light on these interconnected histories.

Our investigation was further enriched by the participation of esteemed researchers Prof. Helena Filipsson from the Department of Geology, Lund University, and Prof. Thomas Laepple from AWI, Helmholtz Centre for Polar and Marine Research, Germany. Their insightful lectures on Hulu Cave's significance provided invaluable context to our fieldwork.

Educational & Collaborative Impact

The field trip fostered vibrant discussions on sedimentation processes, karst formation, paleoenvironmental reconstruction, and analytical techniques for speleothem dating. It also provided an immersive experience for international students and early-career researchers in the real-world field of geology and archaeology.

Conclusion

The Hulu Cave stands as a geological, climatic, and anthropological treasure trove. Its complex history, from karst formation to human occupation and monsoon signal preservation, makes it a multidisciplinary beacon for earth scientists and archaeologists alike. This field trip not only enriched our academic understanding but also strengthened global scientific collaboration on topics of climate change and human prehistory.

Further Reading:

• Cheng, H., Edwards, R. L., Southon, J., Matsumoto, K., Feinberg, J. M., Sinha, A., ... & Wang, Y. J. (2018). Atmospheric $^{14}C/^{12}$C changes during the last glacial period from Hulu Cave. Science, 362(6420), 1293-1297.

• Nanjing Museum and Peking University. (1996). Locality of the Nanjing Man Fossils 1993-1994. Cultural Relics Publishing House, Beijing, 306pp.

• Wang, Y. J., Cheng, H., Edwards, R. L., An, Z. S., Wu, J. Y., Shen, C. C., & Dorale, J. A. (2001). A high-resolution, absolute-dated Late Pleistocene monsoon record from Hulu Cave, China. Science, 294(5550), 2345-2348.

• Wang Y.J. et al. (1999). TIMS U-series ages of speleothems from the Tangshan caves, Nanjing. Chinese Science Bulletin, 44: 1987-1991.

• Wu R., Li, X., Wu, X, and Mu, X. (2002). Homo erectus from Nanjing. Jiangsu Science and Technology Publishing House, Jiangsu, 316pp.