Natural bridges are formed when portions of rock are naturally removed to create a hole, leaving the rock on top still intact. These bridges show the power of nature and the environment on all of its surroundings. Given time and the right circumstances, water and air can shape just about anything, including rock. There are several different types of natural bridges, and all can be found throughout the world.
A natural arch is made of rock and must be exposed to the air. It cannot be submerged in water, or surrounded by soil. Natural arches appear to have one side come out of the air, form a parabola type of shape, and then come back down into another part of the ground. The hole must also go all the way through the rock, in order for it to be constituted as an arch, and the hole must have been created by natural causes, such as from water or wind. The erosion process is the only natural process that qualifies to make an arch. Movement of other rocks to create a hole do not count.
Arches are still called arches even if humans have effected them in some way, but only if the hole was originally created by natural processes. The arch should be intact. It is allowed to have some cracks, but still must look like an arch. There are many natural arches around the world; an exceptional one is located in Springbrook National Park in Queensland, Australia.
A waterfall natural bridge is only present if it is over a current or former stream bed. The bridge is originally formed based on subterranean stream piracy. This is when some of a flowing stream's water gradually leaks into cracks of rock adjacent to the stream bed, eroding away at the stone to create a new main path for the stream to take. A waterfall is normally upstream from the bridge, but depending on how much erosion has occurred, and how mature the bridge is, the waterfall might be very close to the natural bridge. This occurs when the water erodes the rock beneath it and draws back to where the bridge exists. Waterfall bridges are the only type of natural bridge in which water is the sole reason for its formation. These types of bridges can be found in places like Chinle, Arizona.
The meander type of natural bridge exists with a currently active stream of water. Most of the time, these bridges are flat on top and have an arch underneath to let the water pass through. Water is a main cause of the hole, but a wall collapse is also a key factor in its formation. As streams meander around rock, they will create a wall. With more erosion, a wall will collapse and form an oval, then with further erosion, the semicircular shape will emerge. These types of bridges can be found in places like National Monument, Utah.
