The geology of Trou Al'Wesse



Formation of the cave of Trou Al'Wesse

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The cave was formed in a karstic system by infiltration of water through the dolomite. Limestone and dolomite are rocks soluble by water containing carbonic acid (H2CO3) and organic acids. Rain is enriched in carbonic acid in the atmosphere; passing through the earth's soil, additional carbonic acid and humic acids from organic remains are added. The subterranean water then dissolves limestone and dolomite. When the rock is dense enough, water must follow the diaclases, stratification planes and cracks, thus forming larger fissures and cavities, instead of spreading evenly throughout the rock. At Trou Al'Wesse, this process may have begun by subterranean streams created from fissures on the Condroz Plateau, creating the corridors and the chimney at the back of the cave. Water exited by descending toward the Hoyoux River, thus forming the large cavity of the cave.


Next, sediments were transported wind or water to fill the cavity. Climate changes affect geological processes: deposition and erosion phases succeeded one another, with new phases of karstic activity, which makes reading of cave stratigraphy a fairly complex matter. The continuity of sedimentation made possible the preservation, followed by the fossilization of archaeological material abandoned by prehistoric groups. Climatic variations recorded in the geological deposits inform on the climate and environment of prehistoric human societies occupying the site. Caves and rock shelters in Wallonia thus constitute a valuable resource for study of the evolution of humanity. When the discipline of archaeology was first developed, and the idea of "troglodytic man" (cave dwellers) was still common, natural sheleters that were easily found attracted the attention of the first archaeologists. Prehistoric archaeology was first established on the basis of the geological deposits found in caves and rock shelters, particularly abundant in Wallonia.



Current formation of stalactites by the dissolution of dolomite.


Topography and geography

The Hoyoux River cuts a winding valley from the Condroz Plateau to the Meuse River (at the town of Huy). The cave of Trou Al'Wesse is located on the right bank of the Hoyous, near the  former village of Petit-Modave.


The cvity opens at the base of a rocky spur around 50 meters from the Hoyoux and 8 meters above the current river. Its altitude is 202 meters above sea level. The entrance of the cave is oriented toward the southwest and the cave itself is a large, relatively horizontal, gallery about 35 meters long. At the end of this gallery, the ceiling is pierced by a vertical chimney joining the exterior 9 meters above.


The area attracted prehistoric humans because it offered several resources necessary for survival: shelter, access to water, a wide variety of flora and fauna in the valley and on the plateau, and flint available locally and on the Hesbaye Plateau.


Geological analyses


Understanding of the sedimentary dynamics of a site remains an essential approach to the study of the archaeological record itself. This type of analysis is ideally prerequisite to the extension of an excavated zone, but working conditions vary from one project to another. In practice, it is not always possible to conform to an ideal methodology. In this context, the principal contribution of geology is situated in analysis of the stratigraphic sequence and modes of deposition and alteration. Various specific analyses serve as tools for both archaeologist and geologist, from direct observation in the field to the study of micro-grains of sediment in the laboratory.


  • The main objectives of sedimentology are to clarify the nature, origin and mode of deposition for the deposits of a stratigraphic sequence.

  • Geomorphology studies the external aspects of geological processes, at several scales of analysis.

  • Micromorphology studies, via microscope, the internal structure of sediments and reconstructs the succession of events leading to their formation.

  • Geochemistry focuses on analysis of the formation of the earth's crust and internal physical changes that continue to operate today.

Collection of sediment samples for analysis.


The stratigraphy of Trou Al'Wesse

In profile J/K 6-9 (above), the succession of Holocene (brown to dark brown) and Pleistocene (yellowish) strata is clear. The Holocene strata contains Early Neolithic and Mesolithic occupations (stratum 4a: lighter brown; stratum 4b: darker brown). The deposition of the Holocene deposits eroded the sub-horizontal Pleistocene strata. The Pleistocene strata visible in this profile are archaeologically sterile, but contains abundant fauna and microfauna that permit analysis of the environment. Lower in the sequence (not visible), Aurignacian and Mousterian occupations are present in strata 15b and 17a-c, and excavation will begin in these levels in 2006.


Stratum 4a light brown, sandy silt Presence of ceramic sherds and probable domesticated fauna Early Neolithic
Stratum  4b, facies alpha-delta dark brown, sandy silt Concentrations of lithic artifacts, abundant bone fragments and charred remains Early to Recent Mesolithic
Stratum 15b     Aurignacian
Stratum 17 a-c     Mousterian

For more detailed information on the sedimentological analysis of the stratigraphic sequence, please consult:

 PIRSON St., 1999, Étude sédimentologique préliminaire au Trou Al'Wesse (Modave, Belgique). Bulletin des Chercheurs de la Wallonie XXXIX, p. 115-177.

PIRSON St. & COLLIN F., 2005, Contribution à la stratigraphie du Trou Al'Wesse à Petit-Modave (comm. de Modave, prov. de Liege). Notae Praehistoricae 25:39-47.


Page updated 16 March 2006