Les rapides de Lachine EarthCache
Hidden : 9/10/2014
Difficulty:
2 out of 5
Terrain:
2.5 out of 5

Size: Size: other (other)

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Geocache Description:

This Earth cache is about the Lachine Rapids. You can see the rapids from several spots in the Lachine-LaSalle area, but to successfully log this Earth Cache as found, visit the posted coordinates within the Parc-des-Rapides and send me the answers to the questions that you will find below. The Parc-des-Rapides is a public park that is open daily from 7h to 22h. There is free parking available. Please stay on the paths and observe the signs.


Photo: Heritage Montreal


Earth Cache information

The Saint Lawrence River/Flueve Saint-Laurent

At the posted coordinates you will be looking at the Saint Lawrence River (Fleuve Saint-Laurent). It is also known as Kaniatarowanenneh (big waterway) in Mohawk. That’s a good name because it is indeed a big waterway that flows for 1197km from Lake Ontario—the most Eastern of the Great Lakes--to reach the Gulf of Saint Lawrence where it empties into the Atlantic Ocean. Figure 1 shows the Saint Lawrence River system.
Fig. 1 The Saint Lawrence River System today


As it flows, the Saint Laurent widens in three places forming three fluvial lakes. Lake Saint Louis/Lac Saint-Louis is one of these three fluvial lakes. The Lachine rapids begin past the Eastern end of Lac Saint-Louis and continue along the river for about 4.8km towards the South Shore (see Fig. 2).

Fig. 2 Location of the Lachine Rapids



Rapids in General

Rapids generally occur along the course of a river at a point where the riverbed has a relatively steep gradient that results in an increase in water velocity and turbulence. The river, at the point where the rapids are found, is relatively shallow and some rocks are exposed above the surface of the flowing water. As the fast flowing water splashes over the rocks, air bubbles are incorporated and the surfaces takes on a white appearance—i.e.,“white water”. Rapids occur where the riverbed material is more highly resistant to the eroding effects of the water compared to the resistance further downstream.


The Lachine Rapids in Particular

The Lachine Rapids may be the most spectacular of a series of several rapids found along the Saint Lawrence River. This is not surprising when we consider the gradient (change in height above sea level) along the course of the river. The river drops over 68.9m (226ft) between it’s beginning (at the end of Lake Ontario) and the end of Montreal Island (see the gradient in figure 3). The descent over the 4.8km length of the Lachine rapids is about 13.7m (45ft). The formation of the Lachine Rapids can be explained by considering the history of the bedrock of the riverbed and the water flowing over it.



Fig. 3 The change in water levels along the Saint Lawrence today


The bedrock of the river includes statigraphical units (or layers) of sedimentary rock that were deposited over what is now Quebec during the ordoviacian period (about 450 million years ago). This includes Trenton limestone and Utica shale. Trenton limestone is a group of dark limestone that is rich in calcite and dolomite and contains grains of light-gray massive crinoidal limestone—which contain skeletal fragments of marine creatures. Utica shale is a more fine-grained sedimentary rock made up of clay with minerals like quartz and calcite that is prone to flaking and erosion. The Saint Lawrence River around the Lachine Rapids descends from limestone to shale.

About 125 million years ago, as the “North American plate” moved Westward, magma (molten rock) flowed—or intruded— into the older sedimentary limestone and shale, to form minor igneous intrusions which are associated with the rapids.

Water has flowed at the posted coordinates since the melting of the last glacier (around 12,000 years ago) and formation of the Champlain Sea. (figure 4).

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Fig. 4 The location of the historic Champlain Sea



As the sea receded, and the land, now free from the weight of the ice, rebounded, the shore-line around the rapids was much higher than it is today and water also followed the historical Saint-Pierre channel. The currents of water carried away unconsolidated material and eroded the floor. The floor of the historic Saint-Pierre channel was formed by relatively resistant Trenton limestone while the lower section of rapids was cut in softer Utica shale which facilitated downward erosion through the bed (downcutting) (figure 5).

Fig. 5 Distribution of Trenton Limestone and Utica Shale


To log this Earthcache as found

1. Visit the posted coordinates and observe the rapids

2. Send me the answers to these four (4) questions by email—do not post the answers in your log.

a) At the posted coordinates, as you face East, are the rapids greater on the right or the left. Why is that?

b) Near the posted coordinates, there is a sign. Explain why this sign is advisable, what is forbidden and the regulation number.

c) What is the elevation according to your gps at this point?

d) Stage 2 is a WP along the path. It is at N45 25.677 W 73 35.344. At Stage 2, describe the rocks you see on the shore. What kind of rock do you think it is? Keep in mind the sign you saw at the posted coordinates.

3. Optionally, photos of you and/or the area are welcome. Please post them with your log.


References

· Wikipedia
· MacPherson, J.B., 1968, The physiographic evolution of the Montreal Archipelago, Canadian Geographier, XII, 4.
· Environment Canada. Saint Lawrence River https://ec.gc.ca
"Saint Lawrence River." Compton's by Britannica. Britannica Online for Kids. 2014.
· Neighbourhood Old Lachine. Heritage Canada. www.memorablemontreal.com
· Macfarlane, J. 1890. An American geoogical railway guide, giving the geological formation at every railway station, with altitudes above mean tide-water, notes on interesting places on the routes, and a description of each of the formations . D. Appleton and Co.

Note
Si vous êtes intéressé à contribuer à une traduction de ce cache en français, s'il vous plaît contactez-moi.

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