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

http://library.thinkquest.org/C005280/geoformation.htm

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 Canyon Canyon 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 ReefMuley Twist Canyon, Capitol Reef

References www.wikipedia.org http://www-geoimages.berkeley.edu/Waterfalls/ZionCanyon/TempleSinawa

vaL.html http://www.zionparkmotel.com/images/zion-national-park.jpg http://www.terragalleria.com/images/np-plateau/zion0787.jpeg http://www.naturalarches.org/archinfo/faq.htm#whatis http://darkwing.uoregon.edu/~mobar/images/capreef.jpg http://www.americansouthwest.net/utah/photographs700/capvalley.jpg http://www.so-utah.com/archcany/arches/delicate.jpg http://www.travelwest.net/parks/zion/images/zion-canyon.jpg http://www.aztecdaytrips.com/images/rooftop.jpg http://www.computersforart.org/create/arch/big/computer_arch_07.jpg