The Earth Story

Waterpocket fold This image captures one of the dominant geologic features of Capitol Reef National Park and was shared by the US Geological Survey. This is the Strike Valley Overlook, which has a view along the layers on the side of Waterpocket Fold.

Waterpocket

The tilting layers in this shot make up one limb of the Waterpocket Fold, the dominant feature of Capitol Reef National Park. They are sandstones, less erodabe than the surrounding landscape, and stick upward through the surrounding sediment. The whole park is nearly 100 kilometers long but only about 10 kilometers wide as it tracks the exposed limb of this ancient fold.

The back and forth migration of the uplifted layers in this shot is a much smaller fold in the full feature. Rocks are never simple, so even with a huge, landscape dominating feature like the Waterpocket fold, there can be small changes in the angle of the fold, leading to second-order features. A small stream and now the Upper Muley Twist Trail cut across the limb of the waterpocket fold at the weakness created by the second-order fold.

-JBB

Image credit: Drew Brayshaw https://flic.kr/p/FJyF37

References: http://bio-geo-terms.blogspot.com/2006/12/monoclines.html http://www.nature.nps.gov/Geology/parks/care/index.cfm https://tmblr.co/Zyv2Js1sfeiJZ

Waterpocket fold

This image captures one of the dominant geologic features of Capitol Reef National Park and was shared by the US Geological Survey. This is the Strike Valley Overlook, which has a view along the layers on the side of Waterpocket Fold.

The Waterpocket Fold is an anticline, a geologic feature formed when rocks are folded upwards. You may note that this is a valley, a surprising landform to find if the rocks have been folded upward, but there is one more detail that controls the shape of the valley we see.

When rocks fold bend, the rocks at the hinge of the fold are put under stress. The rocks at the widest part of the fold are pulled apart and the rocks at the core of the fold are squeezed. Both of these cases – rocks under tension and compression – can cause the rocks to fracture. When cracks form, water can get into the rocks and erode them away.

Large anticlines commonly are found with their cores eroded away because those rocks are fractured and easily eroded, while the tilted hinges remain in tact as exposed, dipping layers, just as seen here.

The exposed rocks are Jurassic and Cretaceous aged sedimentary rocks, marking times from when the U.S. Southwest went from being a desert filled with sand dunes to a time when it was submerged beneath the waters of an inland seaway.

-JBB

Image credit: USGS/Ida & Leo https://flic.kr/p/wYcF4x

Read more: http://www.nps.gov/care/learn/nature/geology.htm