The difference is in how the formation is created.
Narural arches are weather eroded,
Natural bridges are water eroded.
Weather eroded arches begin their formation as deep cracks which
penetrate into a sandstone layer. Erosion occurring within the cracks
wears away exposed rock layers and enlarges the surface cracks
isolating narrow sandstone walls which are called fins. Alternating frosts
and thawing cause crumbling and flaking of the porous sandstone and
eventually cut through some of the fins. The resulting holes become
enlarged to arch proportions by rockfalls and weathering. The arches
eventually collapse leaving only buttresses that in time will erode.
Some natural bridges may look like arches, but they form in the path
of streams that wear away and penetrate the rock. Pothole arches
form by chemical weathering as water collects in natural depressions
and eventually cuts through to the layer below. The rock is typically
a much harder material than the soft sandstone of the arches.
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