A “waterfall” is defined as an area where flowing river water drops abruptly and nearly vertically. Waterfalls represent major interruptions in river flow. Under most circumstances, rivers tend to smooth out irregularities in their flow by processes of erosion and deposition. In time, the long profile of a river (the graph of its gradient) takes the form of a smooth curve, steepest toward the source, gentlest toward the mouth – to the ocean. Waterfalls radically interrupt this curve, and their presence is a measure of the progress of erosion (upstream erosion). A waterfall may also be termed a falls or sometimes a cataract, the latter designation being most common when large volumes of water are involved – like Niagara, Victória, Iaguaçu, Angel, etc.. Waterfalls of small height and lesser steepness are called cascades; this term is often applied to a series of small falls along a river. Still gentler reaches of rivers that nonetheless exhibit turbulent flow and white water in response to a local increase in channel gradient are called rapids.
The majority of waterfalls result from layers of rock composed of different degrees of hardness, or in some cases a abruptly cross of a geological fault. Hard layers are more resistant to erosion. Soft layers are quickly removed. This is visible all around us in the mountains. The type of waterfall, where there simply happens to be a cliff in the way of a river, forms a minority of waterfalls. In this case, over time, the river will cut into the cliff, reducing the gradient of the waterfall, until eventually, only a rapid will remain.
Along a carbonated platform most of the summits are made of resistant limestone and/or dolomite. The softer shale (and other sediments) layers remain protected beneath these resistant caps. If a river flows over a lip composed of a layer of resistant material (like limestone), that lays atop a softer layer of shale, then the water will remove the soft layer beneath the limestone at a faster rate. This results in the caves found beneath many waterfalls. With the erosion progression, some times, a catastrophic collapse occurs, the cave disappears, and the cliff face moves further up the valley. In many cases, the hard-soft layer relationship remains, and the falls simply migrates upstream. This usually creates a canyon in front of the waterfall. As long as the hardness differential remains, the waterfall will persist.
Streams become wider and shallower just above waterfalls due to flowing over the rock shelf, and there is usually a deep pool just below the waterfall because of the kinetic energy of the water hitting the bottom. Waterfalls normally form in a rocky area due to erosion.
In conclusion the waterfalls occur at places in the river where the topography or height changes. Water runs over a ledge and falls usually vertically to the next level of the river. The ledges are usually made of a more resistant bedrock layer, different from the rest of the river's parent material.
Waterfalls are grouped into ten broad classes based on the average volume of water present on the fall using a logarithmic scale. Class 10 waterfalls include Niagara Falls, Paulo Afonso Falls and Khone Falls. Classes of other well-known waterfalls include Victória Falls 9); Rhine Falls and Sutherland Falls (Class 8); Angel Falls (Class 7); Yosemite Falls, Lower Yellowstone Falls (Class 6). Angel Falls (indigenous name: Parakupa-vena or Kerepakupai merú which means the fall from the highest point, in Pemon language in Spanish - Salto Del Ángel) is the world's highest waterfall, with a height of 979m and a clear drop of 807m. It is located in the Parque Nacional Canaima, in the Gran Sabana region of - Bolivar State, Venezuela.
The height of the falls is so great that before getting anywhere near the ground, the water is atomized by the strong winds and turned into mist. The mist can be felt a mile away. The base of the falls feeds into the the Rio Gauya which flows into the Churun River, a tributary of the Carrao River.
In 1841, Charles Lyell, a British scientist and a father of modern geology came to Niagara Falls. From observations of the rocks at the Falls and along the Niagara River, Lyell was able to demonstrate beyond a doubt that the cascading waters had eroded the gorge from the edge of the escarpment at Queenston - Lewiston to its present location – upstream erosion. Lyell's conclusions were supported by an equally distinguished pioneer American geologist, James Hall. His independent studies for the State of New York included the first accurate survey of the rim of the falls to establish a basis for measuring the rate of recession.
The Fervença water fall corresponds to a dramatical change of the hardness of the rocks. Not referred in the geological map, a possible fault could be associated to the geological limit. The Fervença stream that runs slowly and quietly along the Granja do Marquês Quaternary sedimentary plain, suddenly cross a vigorous gorge of cretaceous limestones. The contact between the cretaceous limestones and also cretaceous soft mudstones produces a small canyon immediately downstream.
This cache takes you to the northern Sintra Platform – the so-called S. João das Lampas Platform, very near the Granja do Marquês plain. The water fall is located at GZ, but for a good observation of the geological/geomorfological phenomena you must stay at N 38º 50’,424 ; W 09º 21’,246. As you could imagine during Autumn or Winter, with rain, the fall is much more spectacular…
In order to log your find, your task is answered the following questions:
1- Measure and/or calculate the dimension of the waterfall.
2- Tell us the colour of hard limestone on top of the fall.
3- Finally – not a question! - take a picture of you and/or your GPS’r with the fall behind.
Answers by the e-mail not in the log.(Using my profile to send).
The photo should be placed on the log after confirmation that the answers are correct. Log without photo or log without authorization will be erased.
Please quote the reference/name of the cache in your answers.
Em Português - A cache
Esta Earth cache leva-o até à Plataforma a Norte do Maciço de Sintra a chamada Plataforma de S. João das Lampas, e muito próximo da superfície da Granja do Marquês. A queda de água está no ponto 0, mas para uma melhor observação deste fenómeno geológico/geomorfológico deverá estar posicionado em N 38º 50’,424 ; W 09º 21’,246. Como devem imaginar será no Outono e Inverno, com as chuvas, que esta cascata é mais espectracular…
Para poder fazer o log desta cache, deverá responder ás seguintes perguntas:
1- Meça e/ou calcule as dimensões da queda de água.
2- Diga-nos qual a cor dos calcários mais duros no topo da queda de água.
3- Finalmente – não é uma pergunta! – tire uma fotografia consigo e/ou com o seu GPS’r com a cascata ao fundo.
As respostas deverão ser enviadas por e-mail e não colocadas no log.(Usem o nosso perfil para as enviar).
A fotografia deverá ser anexada ao log depois da confirmação de que as respostas estão correctas. Se fizerem o log sem a nossa confirmação ou sem a fotografia, este será apagado.
Por favor coloquem no log a referência/nome da cache nas Vossas respostas.