Arcuate fractures in a quartzite boulder
This rock is a hard, metamorphosed sandstone (quartzite), with thin beds (layers) showing internal lamination (cross-bedding is well defined in the lowest of the three beds, and subtle in the middle bed). It was probably sourced from a Late Precambrian quartzite formation, a rock type which can be seen today exposed in the Canadian Rocky Mountains (to the west of Calgary).
This well rounded boulder was likely transported by high-energy river flows. During transport it collided with other boulders, and this energetic 'banging together' produced impact marks ("percussion marks"), the dark arcuate (curving, crescentic) marks present (scale: up to approx. 2 cm long). These marks are dark because they are recessively weathered and it looks like they probably contain dirt and lichen. Note how the arcuate fracture marks cross-cut the older lamination in the rock.
In more recent times, someone collected this and other boulders and placed them on a front lawn in my Calgary neighbourhood as part of an ornamental wall. So this is a piece of building stone that reveals multiple stages of its geological history (to those of us who know how to 'read' such clues).
C. J.R.Devaney
Arcuate fractures in a quartzite boulder
This rock is a hard, metamorphosed sandstone (quartzite), with thin beds (layers) showing internal lamination (cross-bedding is well defined in the lowest of the three beds, and subtle in the middle bed). It was probably sourced from a Late Precambrian quartzite formation, a rock type which can be seen today exposed in the Canadian Rocky Mountains (to the west of Calgary).
This well rounded boulder was likely transported by high-energy river flows. During transport it collided with other boulders, and this energetic 'banging together' produced impact marks ("percussion marks"), the dark arcuate (curving, crescentic) marks present (scale: up to approx. 2 cm long). These marks are dark because they are recessively weathered and it looks like they probably contain dirt and lichen. Note how the arcuate fracture marks cross-cut the older lamination in the rock.
In more recent times, someone collected this and other boulders and placed them on a front lawn in my Calgary neighbourhood as part of an ornamental wall. So this is a piece of building stone that reveals multiple stages of its geological history (to those of us who know how to 'read' such clues).
C. J.R.Devaney