Canyons and Arches or: Why erosion is cool Angela Stickle March 5, 2007 Geology of the Desert SW

Canyons are cool Erosion when water runs through dry regions with sedimentary rocks layers of varying resistance Formation due to natural tendency of rivers to reach a base line elevation The larger the height difference between river and inlet into draining body, the more energy it will expend = more erosion! Energy expended in facilitating a speedy drop in elevation

Canyon Forming processes Stream Downcutting Deepens the channel of a stream or valley Removes material from bed or valley floor How fast downcutting occurs depends on base level Lateral Erosion Widens a stream channel or valley Most dominant if stream level is near the base level If Elevation > Base level Downcutting > Lateral erosion San Juan River, Utah

Canyon Forming processes Mass wasting Aka: mass or slope movement “geomorphic process by which soil, regolith and rock move downslope under gravity.” Creep, slides, flows, etc

Canyon Formation Surface stripped by running water Terraces formed in rocks of varying resistance Vertical erosion predominates and alternating cliffs are formed through downcutting

Zion sediment deposition Kaibab (Upper Permian), limestone Moenkopi (lower triassic, 230 mya), mudstone, shales, siltstones Chinle (upper Triassic), includes petrified woods etc Moenave (Lower Jurassic), siltstone, mudstone, fine sandstone Kayenta (Lower Jurassic), sand and siltstone

Zion sediment deposition Navajo (Low to Mid Jurassic), sandstone Temple Cap (Middle Jurassic), clay and silt Carmel (Middle Jurassic), limestone Dakota (Lower Cretaceous), sandstone

Zion and Kolob Canyons This area includes 9 known exposed formations in Zion Represents 150 MY, mostly mesozoic aged, sedimentation Kolob Canyons from end of Kolob Canyons Road, showing red- orange Navajo Sandstone

Zion National Park Varying depositional environments: Warm, shallow seas Streams and lakes Deserts and near shore environments Uplift of the Colorado plateau exposed sediments to erosion Much later, lava flows and cinder cones covered part of the Zion area

Zion National Park Elevated plateau with sedimentary formations dipping gently east Oldest strata exposed along the Virgin River Youngest strata exposed in Kolob Canyons Canyons in Zion Nat’l Park

Zion canyon About 6000 feet of sediment have been removed from Dakota Sandstone Continuing erosion of Navajo sandstone At head of Zion canyon, Virgin River reaching Kayenta Formation, eroding the shale and undermining the overlaying sandstone Causes collapses Widens Canyon Geologists estimate the Virgin River will cut ~1000 ft before it loses ability to transport sediment Temple of Sinawava

Zion Canyons Zion CanyonCanyon Overlook

Natural Arches “A rock exposure that has a hole completely through it formed by the natural, selective removal of rock, leaving a relatively intact frame.” Must be made of exposed rock! Actually have a hole… Hole formed from natural removal of rock (typically erosion, but lava flow happens too..)

Natural Bridges Natural Bridges are types of Natural Arches Distinguished by: A current of water (e.g. stream) formed the hole or is running through it It is being used as a bridge Looks like a bridge

Several types Alcove Natural Arch Buttress Natural Arch Meander Natural Bridge Pothole Natural Arch Shelter Natural Arch Waterfall Natural Bridge

Alcove Natural Arch Occurs on or next to a vertical cliff face Only forms where vertical joint exists behind and parallel to face Very common, relieves stress Kolob Arch in Zion Eroded into Navajo sandstone Span: 287’, second largest in the world!

Meander Natural Bridge Associated with active stream or streambed Opening in a semicircle and the lintel is arched Wall collapse also important feature (creates semi-circle aperture) Rainbow Bridge Largest natural bridge in the world Opening 246’ high Lake Powell, Utah

Lateral Stream Piracy Two water courses separated at a point by a thin rock barrier Barrier must be breached!! Water of one stream is ‘stolen’ by the other Two occasions: Neck of an incised meander Two tributaries running closely parallel

Capitol Reef Canyons

Capitol Reef Arches Hickman Bridge, Capitol Reef Muley Twist Canyon, Capitol Reef

References geoimages.berkeley.edu/Waterfalls/ZionCanyon/TempleSinawavaL.html geoimages.berkeley.edu/Waterfalls/ZionCanyon/TempleSinawavaL.html