My life as a cat

  High places have always fascinated people, especially when the land has been thrust up from an otherwise unspectacular landscape. This is the case with the Niagara Escarpment, standing high above the relatively flat land between the Great Lakes. As the highest place in the area, the escarpment undoubtedly played an important role in the stories and beliefs of the native people. Even today we wonder how the sheer cliffs of the escarpment came to be.

     Since the escarpment towers so high above us it is tempting to think that some stupendous force within the earth has thrust these cliffs towards the sky. As appealing as this explanation may be, there is no evidence to support this idea. The major break in the crust of the earth necessary for that type of movement does not exist. The rocks underneath the escarpment are not broken and shifted. To understand the origin of the Niagara escarpment we must look far back into the geological history of this area.

     The story starts over 500 million years ago. From then, until about 100 million years ago, the interior of North America was frequently covered by shallow seas. Since North America was located further south for most of this time, the environment was quite tropical and animals like corals and other shelled organisms thrived in these seas. When they died their shells and skeletons accumulated as thick layers of sediment made mostly of mineral calcite (calcium carbonate). During some periods, large amounts of sand and clay sediments were washed off the land and accumulated as layers in the sea. Over time a layer cake-like accumulation of flat-lying layers of sediment built up, compacted, recrystallized and turned to stone.

The layers made of the shells and skeletons of the ancient sea animals become the strong rocks known as limestone and dolostone. You have to look very carefully to find fossilized remains in these rocks today because the sediment recrystallized as they change to stone and the fossil shapes were largely destroyed. The layers of sand, which are mostly composed of the mineral quartz, harden to become a very strong rock called sandstone. You can still feel the grains of sand on a freshly broken surface of this rock. The layers of mud hardened to become a weak and crumbly rock called shale.

When the sea finally left the interior of North America about 100 million years ago, during the time of the dinosaurs, the layer cake of strong and weak rocks covered much of the continent. As long as the rocks were below the sea, they were protected from erosion. Once they were above the sea level and exposed to wind, rain, flowing weather, frost and ice they began to break down (weather) and were carried (eroded) away, sediments again, by steams and glaciers.

How did this give us the Niagara Escarpment? Remember that the great layer cake of rock has strong and weak layers (see Figure 2, a). When weathering and erosion exposed the edge of the rock layer cake, the weak layers were broken down and carried away more quickly than the strong layers. When a thick strong layer has weak layers of rock beneath it, the weak layers will be removed by water, ice and wind leaving the strong layer unsupported (see Figure 2, b). Eventually the overhanging strong layer breaks off and falls, forming a cliff or escarpment (see Figure 2, c). Over time the pieces of the strong rock will be broken up and carried away. More of the weak layers will then be removed until the strong layer collapsed again. This cycle of erosion and collapses is repeated over and over and the escarpment gradually retreats in the direction of the erosion.

Figure 2. A series of sketches showing escarpment retreat from a to c.

That process has been going on in this part of Canada since the time of the dinosaurs. Thick, strong layers of limestone and dolostone (Lockport Formation) form the upper part of the Niagara Escarpment. They are underlain by thick layers of weak rock mixed with thin, easily broken layers of sandstone. Starting much farther north of here, erosion of the weak layers and collapse of the overly strong rock has produced the Niagara escarpment and caused it to move gradually to where we see it today. The process was very slow; taking place under environmental condition that gradually changed from quite warm too much colder as North America slowly drifted north. Under the present environmental conditioned, the escarpment is stabilized by vegetation and is not retreating much, barring the occasional rock fall.

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