1. W. Erskine Johnston Elementary School
Glacially sculpted Precambrian gneiss that displays foliation, folds and cross-cutting dykes
W. Erskine Johnston Elementary School
50 Varley Dr., Kanata, Ont.
Precambrian gneiss shaped by glacial erosion. W. Erskine Johnston Elementary School, Kanata, On.
Photo by J. Aylsworth
Several glacially scoured outcrops of Precambrian gneiss (metamorphic rock) are exposed just beside the school's temporary classroom buildings. The Precambrian gneiss belongs to the Grenville Province of the Canadian Shield, is over 1 billion years old, and represents the oldest exposed rocks in the Ottawa area (often referred to as "basement' rocks" to the younger Paleozoic rock and Quaternary sediment, which have been eroded from this particular area). These rocks, and those of the Gatineau hills in Québec, represent the 'roots' of an ancient mountain system that was once as impressive as the Himalayan Mountains today. One billion years of erosion have gradually destroyed these mountains, leaving the bedrock of their 'roots' exposed. The school is located on the southeastern end of the Carp Ridge - a fault-bounded block that exposes Precambrian rocks that have moved upward relative to the Paleozoic rock elsewhere under Ottawa.
The current shape of the outcrop is the result of glacial erosion during the last ice age. Abrasion and plucking of the bedrock surface under the moving ice sheet has produced a classic glacial landform known as a roche moutonnée. These landforms have a smooth sloping, up-ice side (result of abrasion), and steep, irregular, down-ice side (result of plucking), which indicates southward movement of the Pleistocene ice at this locality.
The coarse-grained gneiss displays a metamorphic layering (foliation) along which the minerals are aligned. Gneiss mineralogy includes quartz, feldspar, mica, hornblende and garnet. The foliation has been folded during deformation of the gneiss, which has also caused some minerals to recrystallize and grow parallel to the fold axes (= mineral lineation). Also evident in the outcrops, are coarse-grained pegmatite dykes of igneous origin (mainly granite, consisting of quartz and alkali feldspar), which cut across the gneissic foliation. These cross-cutting, intruding relationships indicate that the dykes formed after deformation of the gneiss. There are also a few finer-textured dykes of aplite (sugary-textured granite) and diorite composition (dark grey due to more mafic minerals).