Holding Court

This is the second in a series of posts about Zion National Park

Zion Canyon is a canyon through which the Virgin River – a tributary of the Colorado River – runs. All of what I explored during my trip to Zion National Park was in Zion Canyon, only a small fraction of the park itself.

In April, Zion Canyon Scenic Drive – the road through Zion Canyon – is only open to the park shuttle which runs from the Visitor Center (stop 1) in the South to the Temple of Sinawava (stop 9) in the North. Stop number 4 is called the Court of the Patriarchs, after a grouping of rock towers named the Three Patriarchs (after the biblical Abraham, Isaac, and Jacob).

Three Patriarchs (Abraham to the left, Isaac in the center, and Jacob on the right in the background) and Mount Moroni (right foreground).

The Three Patriarchs are composed of a rock unit called the Navajo Sandstone. If you recall the first post in this series – about Checkerboard Mesa – you’ll have heard of Navajo Sandstone. Navajo Sandstone is at least about 200 million years old but may be slightly older and was deposited over a period of about 5.5 million years. It is a sandstone (sedimentary rock) and it is the thickest stratigraphic unit in Zion National Park. The major rock layers are represented by a stratigraphic colum shown below.

Stratigraphic column depicting rock layers that make up formations in Zion National Park. The layers are stacked in order of deposition with the uppermost layers being the youngest (deposited last) and the lowermost layers being the oldest (deposited first). Also depicted are the types of fossils and sedimentary structures that can be found in the units. The diagonal lines in the Navajo Sandstone represent cross-beds. For more information on the Navajo Sandstone and other units see this NPS page from which this column was copied.

As mentioned in the above caption, the Navajo Sandstone is known for its prominent large-scale cross-bedding. So what is cross-bedding exactly? Well, it’s a sedimentary structure (feature of a sedimentary rock) that forms when layers develop at an angle with the primary bedding plane. An example of this in the Navajo Sandstone is given in the picture I took while hiking Angel’s Landing Trail in Zion National Park.

An example of cross-bedding in the Navajo Sandstone along the Angel’s Landing trail. The horizontal lines are the bedding planes and the diagonal lines are the cross-beds. The cross-beds result from sand dunes deposited and shaped by wind flowing right to left in this case.

The Navajo Sandstone represents a remnant of an ancient field of sand dunes in a desert that existed during the early Jurassic – about 200 million years ago. This vast desert landscape extended across the Colorado Plateau region and beyond, approximately spanning from eastern CA to central NM and from Idaho to the Mexico border. This was the largest known sand desert in the history of our planet!

Cross-beds from sand dunes are significant because they tell us about the depositional environment; namely, the direction of transport for the sand grains that eventually formed the rock. For more about this see the diagram below.

A diagram depicting the formation of sand dunes and associated cross-beds. The cross-beds point downward in the direction of wind travel (left to right in this case). Credit: NPS

Stay tuned for more on Zion National Park and its geologic wonders!

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