Abstract:Objectives: Periglacial landforms, including block fields (felsenmeer), rock glacier, stone circles, and patterned ground etc., are the key elements reflecting the past and even recent climatic and environmental conditions. They are often found in the past active glacial landform areas, and easily affected by modern mass wasting events like debris flow and rock fall etc. However, our Earth scientists often misunderstand these landforms due to lack of criteria to recognize the phenomena. To clarify a standard criteria for identifying periglacial landforms, we tried to dissect a block field to reveal its profile.Methods: We selected a large block field, Wulanba block field, in the north of Saihanwula National Natural Reserve, Chifeng, Inner Mongolia, to ditch and reveal the profile structures, and tried to reconstruct the past periglacial environment. Furthermore, we correlated the exhumed profile in Saihanwula to Lanmaqiang block pile in Mount Mengshan, and confirmed the Lanmaqiang block pile as a boulder stream formed in the early Holocene.Results: The profile exhumed at the eastern margin of Wulanba block field shows the following features:(1) The eastern margin of Wulanba block field is composed of angular andesitic porphyrite blocks, building an active layer as thick as 1.8m above the bedrock head.(2) The rock blocks pile up in particle supported texture, leaving the voids without matrix inside. This is called openwork structure.(3) The larger blocks, including some more than 50cm in diameter, are floated in the upper part, while smaller particles and rubbles dropped to the lower part of the profile with the clays and soils filled in the small voids of the rubbles directly above the bedrock head.(4) The exhumed part of the block field belong to inactive block field. However, there are some patches of active block fields above 1800 m a.s.l. in the Natural Reserve, revealing some isolated permafrost remaining in the local periglacial envrironment.Conclusions:(1) We dug a pit at the margin of Wulanba block field in the Saihanwula National Natural Reserve to show the structure of an intact field block. The openwork structure of the large angular blocks and small rubbles can be a unique feature to identify block field (or felsenmeer) or block stream formed in the periglacial condition.(2) According to the block field formation under freezethaw conditions, we estimated that the climate in Saihanwula has warmed up for at least about 2~6°C since early Holocene.(3) We correlated the excavated block field profile in Saihanwula to Lanmaqiang boulder pile in Mount Mengshan, and found the similarity of the two, confirmed that the Lanmaqiang boulder pile was a boulder stream formed in periglacial environment in the early Holocene.(4) Former researchers argued that Lanmaqiang boulder pile was a lateral moraine or debris- flow deposits. We inspected landforms in the vicinity area of Lanmaqiang boulder stream and concluded that there was no evidence or traces for lateral moraine or debris- flow deposits.(5) Block field and other related periglacial landforms are very closely related to former glacial landforms and modern mass wasting events, so we have to investigate these landforms and understand their mechanisms.