A gorge is a narrow valley with steep, rocky walls located between hills or mountains. The term comes from the French word gorge, which means throat or neck. A gorge is often smaller than a canyon, although both words are used to describe deep, narrow valleys with a stream or river running along their bottom.
A number of natural forces form gorges. The most common is erosion due to streams or rivers. Streams carve through hard layers of rock, breaking down or eroding it. Sediment from the worn-away rock is then carried downstream. Over time, this erosion will form the steep walls of a gorge. The flooding of streams or rivers increases the speed and intensity of this erosion, creating deeper and wider gorges.
Geologic uplift also forms gorges. Geologic uplift is the upward movement of the Earths surface.
Erosion and geologic uplift often work together to create gorges. Parts of streams or rivers can be elevated, along with land, during the process of geologic uplift. As rivers or streams flow across this uplifted surface, waterfalls form. Over time, the power of the waterfall erodes the softer rock layers underneath, causing the original river bed to collapse and create a gorge.
The movement and melting of glaciers can also produce gorges. Glaciers cut deep valleys into the Earths surface. These rivers of ice can create huge canyons and sharp, steep gorges. As glaciers melt, or retreat, these gorges and canyons are exposed.
As the river erodes the rock a waterfall can form.
Waterfalls are found in the upper course of a river. They usually occur where a band of hard rock lies next to soft rock. They may often start as rapids.
As the river passes over the hard rock, the soft rock below is eroded more quickly than the hard rock leaving the hard rock elevated above the stream bed below.
The 'step' in the river bed continues to develop as the river flows over the hard rock step as a vertical drop.
The drop gets steeper as the river erodes the soft rock beneath by processes such as abrasion and hydraulic action. A plunge pool forms at the base of the waterfall. This ersosion gradually undercuts the hard rock and the plunge pool gets bigger due to further hydraulic action and abrasion. Eventually the hard rock is unsupported and collapses. The rocks that fall into the plunge pool will continue to enlarge it by abrasion as they are swirled around. A steep sided valley known as a gorge is left behing and as the process continues the waterfall gradually retreats upstream.
Information obtained from: national geographic, wikipedia and geology.com
On site questions.
#1 What colour is the rock that make up the walls of the gorge?
#2 what colour is the rock that make up the bottom of the gorge?
#3 Has the waterfall receded in the past? Validate your answer.
#4 In your own words, how do you think the gorge formed and why you say so.