Characteristics of the stone from the "Carrière sur les Roches".
At “LOS ANGELES”: stone hardness classification: 1st category, just below precious stones.
At “Micro-Deval”: Classification of stone wear: 2nd category
Density: 2.80
Absorption rate: Between 0.2 and 0.4
Moisture content: —————
Gelifraction: Non gelive
Visit
- a report by Mr Grogna, the quarry’s geologist.
- Stone analysis reports.
Report courtesy of Mr. Grogna, Quarry Geologist.
GEOLOGICAL DATA – “SUR LES ROCHES” quarry in Bastogne
The “Sur les Roches” quarry in Bastogne belongs to the Eifel Synclinorium. It mines psammopelite formations from the Lower Devonian.
The rocks at the quarry are commonly referred to as “sandstone”. They are made up of alternating multi-centimetric to multi-decimetric banks of rocks defined by specialists as quartzophyllic, which, depending on the banks, may be zonary, granitic, quartzite or argillaceous, sometimes with argillaceous joints resulting from alteration. By other names, the rocks can be defined as feldspathic or arkosic quartzites, sometimes chlorotic, chloritized feldspathic psammitoquartzites, zoned psammites and micropsammoschists, sometimes feldspathic.
The tectonic structure is an anticlinal fold in a general NE-SW direction, accompanied by secondary folds. Longitudinal undulations give it a complex character.
The quarry is located in the axial zone of the fold. In the SE wall, the general direction of the flank is NE-SW with a dip of around 30°. In the NW wall, the other flank has a similar direction and dips approx. 15° to the N. The position of the benches is highly variable in the hinge of the major fold and in the secondary folds. It is, however, easy to follow within the perimeter of the site.
The facies is heterogeneous vertically. Laterally, however, it is quite homogeneous. Layers are generally regular. Minor local irregularities are due to folding and variations in sedimentation rates.
Fracture systems are numerous and some fractures are large, with a frequency of around 50 cm, often filled with quartz and “bastonite” (black mica). Diaclases parallel and orthogonal to the stratification are frequent and sometimes filled with quartz. The geological term used to define this particular structure is “BOUDINAGE”.
Bench-to-bank sliding is rare and not very pronounced. Slight crushing occurs in the hinge zones of major and secondary folds, but has no effect on the strength of the crushed rock. The presence of fillings in the majority of fractures and the more argillaceous layers severely limit the rock’s permeability.
The quarry produces a variety of high-quality aggregates that meet current quality standards. These aggregates are intended for the local market, including the nearby Grand Duchy, and are used in high-quality concrete, asphalt mixes, ballast, foundations and drainage sub-bases.
As an anecdote, the scientific term “BOUDINAGE”, adopted worldwide, was defined in a quarry in Bastogne during a geological excursion over a century ago. Belgian geologists observed quartz mineralization intersecting certain rock benches at fairly regular intervals, resembling boudins. After studying the phenomenon, they found that these quartz mineralizations were only found in the competent (hardest) beds, while the other, more incompetent (softer) layers directly above and below were unaffected. In fact, it was an ancient tectonic phenomenon that caused a series of fractures in the competent layers (parallel to the surface of the benches), while the more incompetent neighbouring layers were allowed to stretch without breaking (creep). The fracture zone was filled with quartz, whose characteristic milky color stands out against the bluish-gray rocks.
Internet references on boudinage:
ttp://en.wikipedia.org/wiki/Boudinage
http://popups.ulg.ac.be/ASGB/docannexe.php?id=1863
* GROGNA Joseph
* Consulting engineer – Consultant in the mining and quarrying sector.