The Geomorphology of the Gorge

The geology of the falls and the gorge is a concept with many details that can be difficult to explain.

Around 430-440 million years ago, an oceanic basin made its way into our region. This basin extended to our neighbors that is, to northern Maine in the United States and to the Gaspé Peninsula.

On the bed of the basin, there were two types of sediment: clastic (sand, silt, and siliceous minerals resulting from erosion of the banks by waves and wind) and carbonate (resulting from the decomposition of organic matter such as dead animals and plants).

These sediments deposited over millions of years on the plateau, slope, and bottom of the basin, thus forming sedimentary layers.

These sediments, which are the size of clay particles, have the characteristic of being very unstable materials, so a simple movement of the Earth's crust is enough to cause a landslide towards the bottom of the basin.

The weight exerted on the sedimentary layers from the bottom of the basin increased (due to mudflows), thus favoring the formation of compact rocks (the weight of upper layers on the lower layers promote the extraction of water from the mud). These landslides are the origin of the numerous folding zones found, among other things, in the "wells in rock."

However, the structure of the rocky substrate was disturbed 400 million years ago (due to the collision between two continental masses that formed the Appalachians, which extends from Newfoundland to Alabama). During this period, the rocky structure of the basin was compressed, folded, and uplifted.

These rocks were originally sedimentary, but over the years, they have been metamorphosed (transformed), which is why they are said to be metamorphic.

The numerous small cracks found throughout the rock face are actually fracture lines (parallel fracturing caused by forces parallel to the movement of tectonic plates). These folds must have occurred at a specific depth in the lower structural levels.

The white vein-like lines are deposits of calcite (limestone calcium carbonate). These have risen to the surface as a result of dissolution at depth and have settled in the faults and micro-fractures on the hinge of the formed folds.

Almost all of the Canadian landscape has been modified by the various glacial periods that occurred 1.5 to 2 million years ago. The traces left by the glaciers, however, date back only to the last glacial period (the Wisconsin began 80,000 years ago and ended 7,000 years ago), during which time Canada was covered by a giant continental glacier.

Much of the relief has been eroded as a result of these glacial periods, which explains the rounded relief of the region (the Appalachians would have been as high as the Rockies).

Before the glacier arrived, the Saint John River channel had a "V" shape, and because the glacier used the river valley as a "channel" for flow, it took on a "U" shape due to erosion caused by the glacier.

Looking at aerial photos of the region, numerous glacial features can be observed.

About twelve thousand years ago, when the glacier retreated, a vestige of the ice cap remained above northern New Brunswick and Maine. The melting of this cap brought numerous amounts of material into the region (moraine), which are largely settled in the Saint John River Valley.

Following the melting of the glacier, the region underwent isostatic uplift (the land is depressed under the weight of the glacier and rises when it melts, that's a rule!). Isostatic uplift goes hand in hand with rapid river entrenchment (the troughs are the bed!) Such uplift involves a significant period of erosion. With the gathering of the river, we observe the formation of terraces (this is what formed the gorge).

The input of material caused by erosion and melting of the glacier obstructed the southern outlet of the river, particularly due to the low slope, and the northern outlet, due to the water supply from its tributaries (Falls Brook and Madawaska Lake, a glacial lake at the time (now Little River) have carved a channel into the rock (if this had not happened, the falls would not have existed). A carved river is excavated in such a way that its flow is subject to minimal physical constraints.

The principle of gorge erosion is the same as that of Niagara Falls. There is undermining at the base of the falls and collapse of the material forming the fault escarpment.