Arches and Alcoves are formed when Horizontal bedding planes are weathered and eroded back, sometimes it's between the contact of 2 different rock members. A good example is In Arches National Park, the contact between the Dewey Bridge member and the Slick Rock member are the most readily exploited sedimentary rock planes in the National Park.
Slightly Acidic groundwater can dissolve calcite cement, this releases grains of sand and silt. Over time the fin is breached and a horizontal opening is formed.
Gravity-Induced fractures will develop above the breach where the rock above is unsupported and allows blocks of rock to fall from the opening. Weathering continues along with the curved fractures and formes a larger opening along with its iconic shape.
Twisted Doughnut Arch --- Arches National Park - Utah (U.S.A)
The Bearpaw Formation is comprised of Marine Siltstone from the shallow Bearpaw Seaway, which was a portion of the Western Interior Seaway during its last succession. The upper Bearpaw Formation is comprised of mudstone which is characterized by the brown colouring at the contact of the Horseshoe Canyon Formation.
Fine-grained Sandstone in the formation is relatively rare but does exist, as the Bearpaw formation was deposited in shallow seas.
The base layers of the Hoodoos are formed from the Bearpaw Formation, the top layer of the formation is younger towards the southeast as that was the direction the seaway was retreating.
Elbow Arch - Mystery Valley - USA
Hidden Canyon Arch - Zion - USA
Honeymoon Arch - Mystery Valley - USA
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