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Travel My Hometown In L. Travel The last artists crafting a Thai royal treasure. Subscriber Exclusive Content. Why are people so dang obsessed with Mars? How viruses shape our world. The era of greyhound racing in the U. See how people have imagined life on Mars through history. See More. Over time this gets bigger, increasing the size of the overhang until the hard rock is no longer supported and it collapses. This process continues and the waterfall retreats upstream. A steep-sided valley is left where the waterfall once was.
This is called a gorge. V-shaped valleys and interlocking spurs In the upper course there is more vertical erosion. V-shaped valley in West Sussex. This suggests that the hard rock layer is overhanging. Waterfalls such as Steinsdalsfossen in Norway are examples of such a waterfall in the advanced stages of development.
As the undercutting continues, eventually the overhanging hard rock becomes unstable and top heavy. Ultimately, a chunk of that erosion-resistant hard rock layer collapses and falls into the base of the waterfall.
The net result of this action is that the waterfall retreats further upstream to the remaining lip of the hard rock layer. With its high volume of waterfall, Niagara Falls continues to retreat at a whopping rate of about 3 feet per year! In the adjacent photo, look at the overhanging wall in the photograph, which is further evidence that this process is still going on!
The undercutting still continues until you run out of the hard rock layer. At that point, the watercourse will probably go back to being a stream or rapid.
Now while these processes highlight the fact that things as seemingly permanent as waterfalls can come and go over time, these same processes also take many, many years to occur.
Heck, even things that took thousands of years to occur are like a blink of an eye in geologic time scales! In any case, I made an attempt at capturing the waterfall formation process in a single drawing to further help you visualize it. Then, insert a board or anything relatively flat and hard with some degree of thickness the thicker the better somewhere into middle of the side of the sand pile. Finally, pour water onto the slope of the sand pile the same side you inserted the hard, flat item and observe the water flow over the sand and hard object.
If done right, the water should be flowing down the slope of the sand pile eventually cutting into it. Once you accept the idea and science behind the waterfall formation, you might be able to imagine how you can end up with different shapes or types of waterfalls simply by varying the orientation and combination of hard and soft rock with water cutting through them. Now you may be wondering how all the different rock layers got there in the first place and why these rock layers erode at different rates.
Well, just about all of the land you see around you is the result of volcanoes. Volcanoes typically spew out lava, which is really molten rock or rock that is so hot that it acts like a liquid. You can see this action taking place in places like the Big Island of Hawaii, where the lava ultimately solidifies to add to its landmass and become part of the rock layer.
That fact alone illustrates that the rock layers that form from the cooling lava can have differing compositions. And its this differing composition that results in the differing levels of resistance to erosion by water and to a lesser degree wind. Places like the Grand Canyon in Northern Arizona exemplify such geologic episodes as the Colorado River exposed and revealed the many layers of rock holding up the majestic cliffs and buttes.
Each layer represents a change in the geologic conditions. It could be a volcano going off depositing a layer of new lava, or a major event like an asteroid collision depositing a thick soot and dust layer e. The internal heat of the earth also is the engine behind convection, which moves the molten rock beneath the crust.
Some of these plates slide against each other, others collide, and still others separate from each other. There are even hot spots that practically tear heat holes through the crust like the ones that formed the Hawaiian Islands. Bottom line is that these factors are what gives rise to the various rock layers as well as features we see like mountains, valleys, drainages, flatlands, and more!
Below is a simplified diagram perhaps too overly simplified of the water cycle. This is the dominant source perhaps the only source of all inland freshwater. So how does water get into the higher elevations on land?
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