What Makes A Pothole?
The list of ingredients is pretty short. Bedrock. Turbulent water. And the abrasive mixture that geologists call suspended sediment. Start with bedrock. Without it, potholes won't form. Why? Anything less rock-solid will simply be washed away. "Washed away"? That's the key to understanding Ingredient #2. Potholes are carved from rock, and moving water provides the energy. This leaves only the suspended sediment. It provides the cutting tools for the job. Three ingredients: rock, water, sediment. Three roles: medium, muscle, tool. Nature as sculptor.
Let's spend a little more time on water. If you've ever capsized in a big rapid or heavy surf, you've felt the power of moving water. The faster water moves, the more power it has — and the more power it has, the more suspended sediment it can carry along with it. But fast-moving water doesn't flow quietly for long. As soon as it meets an obstacle, it twists and turns, bending back on itself and forming eddies, souse holes, whirlpools, and boils. These macro-phenomena have micro counterparts: as bedrock is roughened by the impact of transported sediments (a process known as corrasion), the resulting turbulent flow generates subaqueous vortices called kolks. These spin round like small tornadoes, plucking material from the riverbed. And that's not all. The "white horses" that make life lively for canoeists and kayakers are also reproduced on a smaller scale beneath the surface. When stream velocities are high, river-bottom turbulence generates bubble trains that subsequently burst with explosive force. Over time, these tiny explosions will pockmark even the hardest rock, in the same way that cavitation pits the surface of high-speed propellers.
This is all it takes. Each new irregularity generates more turbulence, and every increase in turbulence breeds new irregularities. The milling materials — sediments ranging in size from silt to cobbles — then do their work, tirelessly grinding away. Soon a cavity starts to form in what was once smooth rock. A pothole is born.
Of course, what goes up must come down, right? Rivers in flood are no exception. And when the high water recedes, hidden potholes are exposed to view. This is especially true on steep, flashy rivers — like the river that flows past the Niagara Falls.
Ice Age History of the Niagara River
The Niagara River, as is the entire Great Lakes Basin of which the river is an integral part, is a legacy of the last Ice Age. 18,000 years ago southern Ontario was covered by ice sheets 2-3 kilometers thick. As they advanced southward the ice sheets gouged out the basins of the Great Lakes. Then as they melted northward for the last time they released vast quantities of meltwater into these basins. Our water is "fossil water"; less than one percent of it is renewable on an annual basis, the rest leftover from the ice sheets.
The Niagara Peninsula became free of the ice about 12,500 years ago. As the ice retreated northward, its meltwaters began to flow down through what became Lake Erie, the Niagara River and Lake Ontario, down to the St. Lawrence River, and, finally, down to the sea. There were originally 5 spillways from Lake Erie to Lake Ontario. Eventually these were reduced to one, the original Niagara Falls, at Queenston-Lewiston. From here the Falls began its steady erosion through the bedrock causeing many potholes.
However, about 10,500 years ago, through an interplay of geological effects including alternating retreats and re-advances of the ice, and rebounding of the land when released from the intense pressure of the ice (isostatic rebound), this process was interrupted. The glacial meltwaters were rerouted through northern Ontario, bypassing the southern route. For the next 5,000 years Lake Erie remained only half the size of today, the Niagara River was reduced to about 10% of its current flow, and a much-reduced Falls stalled in the area of the Niagara Glen.
About 5,500 years ago the meltwaters were once again routed through southern Ontario, restoring the river and Falls to their full power. Then the Falls reached the Whirlpool. It was a brief and violent encounter, a geological moment lasting only weeks, maybe even only days. In this moment the Falls of the youthful Niagara River intersected an old riverbed, one that had been buried and sealed during the last Ice Age. The Falls turned into this buried gorge, tore out the glacial debris that filled it, and scoured the old river bottom clean. It was probably not a falls at all now but a huge, churning rapids. When it was all over it left behind a 90-degree turn in the river we know today as the Whirlpool, and North America's largest series of standing waves we know today as the Whirlpool Rapids.
The Falls then re-established at about the area of the Whirlpool Rapids Bridge upriver to our right, and resumed carving its way through solid rock to its present location.
The Niagara is a fairly young river, only 12,000 years old!, a microsecond in geological time. At the bottom of the Niagara falls, the water travels 15 miles over many gorges until it reaches the fifth Great Lake-Ontario. The land between the lakes does not slope at an even grade, but forms a spectacular drop approximately the same height as a 20 story building.
What You Need To Do
To Claim This Cache As A Find,
Please E-mail me the answers to the following questions:
1. List the three ingredients that are required to make a pothole?
2. In your own words, how do potholes form?
3. What great lake does the Niagara River empty into?
4. Estimate the height, width, and depth and the largest pothole in the picture above, which is located at the posted coordinates.
5. Estimate the elevation of the pothole shown above, which is located at the posted coordinates.
Important Note -- the Niagara Glen is a Nature Reserve since 1992 that contains wild flora and fauna (plants and animals). Please observe all posted regulations. Stay on the trails, that are marked and follow generally accepted trail etiquette. Fires are not permitted. Take only photos, leave only footprints! Niagara Parks Act Regulations:www.niagaraparksnature.com