Archive for the ‘rocks’ Tag

Death Valley V: Geologic History   Leave a comment

The morning sun hits the Panamint Range, as viewed from Death Valley.

The morning sun hits the Panamint Range, as viewed from Death Valley.

This is the second of three posts on the natural history background for a visit to Death Valley National Park in California.  I hope it sparks some interest in these subjects, because if you visit this desert park, you will be hard-pressed to ignore its stunning geology and arid ecology.

GEOLOGIC HISTORY

The rocks exposed in Death Valley go back nearly two billion years.  As you walk through canyons like Titus or Marble, you will see layer upon layer of a dark gray sedimentary rock (often weathering red to orange).  A great thing to do on a hot day in a canyon is to go into the shade of these walls and lean your whole body against the cool gray rock.  This is limestone, and it tells of a time when this area was covered in a warm subtropical sea.

The famous Artist's Palette in Death Valley as viewed from atop the ridge that is most often photographed.

The famous Artist’s Palette in Death Valley as viewed from atop the ridge that is most often photographed.

Back in Paleozoic time (250-600 million years ago), there was a quiet coastline not far east of here one very similar to the modern Atlantic coast of North America.  Marine algae and other small creatures pulled CO2 and calcium out of the seawater to form their shells. These lime muds accumulated layer upon layer, eventually to become limestone.  Sand, silt and mud covered the shallow marine shelf at times, leading to sandstone, siltstone and shale.

Later, during the time of dinosaurs (the Mesozoic), the whole region was the focus of mountain building, thus emerging from the sea.  And mountain building means plate tectonics.  At that time, the ancestral Pacific Plate (called the Farallon Plate by geologists) pushed underneath the western edge of North America – a subduction zone.

Recently formed salt crystals decorate the floor of Death Valley in California.

Recently formed salt crystals decorate the floor of Death Valley in California.

The incredible pressures generated along this subduction zone made the limestone and other rocks pay dearly for being in the wrong place at the wrong time.  These sedimentary rocks were originally deposited in horizontal layers, and as you can easily see in the naked mountains of Death Valley, they have been folded, faulted, and otherwise tortured.  Masses of granitic magma, melted crustal rocks from below, pushed up into the sedimentary rocks.  This granite is best exposed to the south, in Joshua Tree and other parts of southern California.

A view of Death Valley from above Artist's Palette shows the playa with its salt pan.  A large alluvial fan is at upper left with dark inselbergs emerging in places.

A view of Death Valley from above Artist’s Palette shows the playa with its salt pan. A large alluvial fan is at upper left with dark inselbergs emerging in places.

The spectacular results of this ultra slow-motion collision can be seen on any canyon hike in Death Valley.  In addition, many of the rocks have been changed – metamorphosed – into a wholly different kind of rock.  The uplifted area was slowly worn down by erosion over a long, long time, eventually forming a low plain.  In other words, there were no rocks formed, in this case from the Jurassic to the Eocene, a period of 130 million years!  The missing time interval shows up as an ancient erosional surface in the rocks, what is called an unconformity.

 Unconformities are important horizons in any rock sequence, and this one shows itself in various places across Death Valley.  You can see a textbook example of an angular unconformity (the most obvious kind) in Darwin Canyon.  This canyon is about 19 miles from Panamint Springs (where you’ll ask for directions and road conditions).  It shows as a line in the rocks (surface in 3 dimensions) where layers below are at a completely different angle than those above.  In the same area is some fantastic folding.

Mesquite Flat in Death Valley National Park, California, offers great opportunity to photograph landscapes in black and white.

Mesquite Flat in Death Valley National Park, California, offers great opportunity to photograph landscapes in black and white.

THE BIG RIP

Long after the dinosaurs had disappeared, starting several million years ago, this area began to be torn apart by rifting at the edge of North America.  It’s a process that continues today.  By this time the subduction zone off the west coast had shrunk northward, where it still grinds away off the coast of Oregon and Washington.  It was replaced by the San Andreas Fault, which still marks the boundary between the North American and Pacific tectonic plates.

The lateral sliding movement of the enormous Pacific Plate moving north past the western margin of North America is essentially torquing the entire western part of North America.  It’s caused a clockwise rotation and the crust has broken into large fault block mountain ranges bounded by normal faults.  This rifting (as rifting typically does) opened pathways for lava to rise and erupt.  Throughout Death Valley you will see areas of volcanic rocks – mostly tuff (rock made from volcanic ash) and basalt (dark lava rock).  Ubehebe Crater in the north past Scotty’s Castle is just one example.

The skies above Death Valley are the playground of Navy pilots from nearby China Lake.

One of the only times you’ll look up from the stunning landscape of Death Valley is when a deep boom makes you notice the Navy jet pilots from nearby China Lake, who make the skies their playground.

 The fault-block mountains caused by rifting are Death Valley’s most obvious geological structure.  But in this far southern part of the Basin and Range, you are looking at a deeper level of rifting.  So there are not only the steep normal faults, but also low-angle “detachment” faults.  Think about the steep normal faults that border the mountain fronts curving and taking on more shallow angles as you mentally travel down their surfaces, and you have a great idea of a detachment.

Incidentally, remember the granite formed during the Mesozoic?  Go south, to Joshua Tree and other places in Southern California, and you’ll see the masses of granite all around.  This means you are seeing much deeper levels of the rifting of North America than you see in the northern Basin and Range.  Keep going and you’ll come to the Gulf of California, where the Sea of Cortez has already invaded the rift.  It’s as if a giant zipper was slowly opening, south to north along the western edge of the continent.

A black and white rendition of the simple beauty of Death Valley's sand dunes.

A black and white rendition of the simple beauty of Death Valley’s sand dunes.

 Back to detachment faults: they can cause whole mountain ranges to literally slide down a sort of shallow ramp, ending up miles from where they started.  Tucki Peak may have slid in this manner.  They really are the most efficient way to rip apart a continent!  You can see these large, low-angled surfaces where they help to form the geographic features called turtle-backs.  One such site is about 16 miles south of Badwater, where if you stop at Mormon Point and look north into the Black Mountains, you’ll notice one of these ramp-like detachment faults.

One more post coming to finish up with Death Valley, this one on the Ice Ages and the pup fish.

The golden light of a late afternoon warms the dunes at Mesquite Flat in Death Valley National Park.

The golden light of a late afternoon warms the dunes at Mesquite Flat in Death Valley National Park.

The Grand Staircase II   Leave a comment

In southern Utah, Red Canyon and its colorful rock formations see their first snowfall of the year.

The oldest Grand Staircase’s steps, the Chocolate Cliffs, are mostly Permian in age.  This is the time before dinosaurs, when reptiles and amphibians ruled the world.  But most of the formations in the Staircase were laid down during the Mesozoic Era.  They span the Jurassic and Cretaceous Periods – more than 130 million years!  They are sedimentary in origin, formed in shallow marine and low-lying land environments.  There were river plains, seashores, coral reefs and enormous deserts.  North America was lower in latitude then, and so warmer.  In short, it was during much of this enormous span of time a paradise for the plant and animal life, a time so long before ours that it is impossible to grasp.  

Layers of sandstone are striking as the sun gets lower in the Escalante River country of southern Utah.

And the animals that walked the earth then?  Dinosaurs!  Their fossil bones and tracks are found throughout the Grand Staircase region.  For example you can visit a dinosaur trackway not far from Kanab, just beyond Coral Pink Sand Dunes State Park.  Check at the BLM visitor center for directions.  There are also fossil tracks south of Escalante, off of Hole in the Rock Road.  Drive ~20 miles down the road and turn right on Colette Canyon Road; then look for a large white outcrop (the tracks are on top).  You can get info. at Escalante Outfitters in that town.

Cryptobiotic soil covers the ground in a sandy wash near the Escalante River in Utah.

At the top of the Staircase are the Pink Cliffs, the youngest of the steps.  These rocks date from a time after the dinosaurs went extinct.  Though there are not many fossils from this formation, this was the time when mammals rose to take the place of dinosaurs in all of the ecosystems once dominated by the giants.  So when you stand atop the Paunsaugunt Plateau in Bryce Canyon National Park, looking out and down the Grand Staircase, you are standing at a time when mammals were rising to dominance.

The cross-bedded sandstone in the Vermilion Cliffs National Monument, Utah forms sinuous patterns across the landscape.

 And when you look out to the south and east, down the Staircase, you are looking back in time.  You might think you are merely looking across cliffs and mesas, but you’re actually peering through the immense ages of time when dinosaurs ruled the earth.  Later, at night, look up at the impossibly brilliant stars, and realize you are, again, looking back in time.  The light from the distant stars began its journey across the vastness of space long ago, and so carries evidence of that time through space and time.

The hoodoos (rock towers) in Red Canyon are similar to those exposed in the Pink Cliffs of nearby Bryce Canyon, Utah.

A few little hints on how to recognize the different environments represented in the rocks.  The bold sandstone outcrops that have the angled stripes (layers) are ancient sand dunes.  The striking pattern results when sand grains blow up and over the top of a dune, forming angled layers (cross-bedding) when they tumble down the so-called slip face.  So you are looking at an ancient version of the Sahara.

Hoodoos are a common feature found in the Grand Staircase/Escalante National Monument.

Other sandstone formations of the Grand Staircase also have tilted layers in places but at a much lower angle.  This rock often has thinner platy layers with much silt and clay in between the thicker sandstone beds.  It was laid down by ancient rivers.  Both the ancient dune sands and ancient river sands make rock that is mostly tan in color, but iron oxides can give it a reddish hue, especially on the weathered surfaces.  Thinly layered siltstone and shale often forms vegetated slopes in this arid region, not cliffs.  So these formations are easy to miss.  You can see the small platy pieces in the soil if you look where you’re walking.  These rocks were formed in a shallow marine environment, or in bays close to the coast.  Think mudflats and deltas and you have the idea.

Calf Creek just above its confluence with the Escalante River has numerous potholes from infrequent floods, and which show up during autumn’s low water flows.

 

Limestone, unlike in the Grand Canyon, is not a huge part of the Grand Staircase, but it is here.  It is formed in and near coral reef areas, or further from the coast in deep water.  This rock is often gray in color, though it can often show red on the weathered surface.  What you will notice about limestone in an arid environment is that it is abrasive, like a very coarse sand paper.  Hiking in limestone terrain, every geologist knows, can really chew up a pair of boots.  Occasionally in this region, not often, you’ll notice lava rock.  Basalt is the most common type.  It’s quite dark in color, often has holes in it (from gas bubbles) and contrasts greatly with the light-colored sandstone.

In Grand Staircase National Monument in southern Utah, the first of winter’s storms brews to the west.

This Monument is huge!  It’s tough to decide where to spend your time, especially when that time is limited.  The next post, my last on the Grand Staircase, will cover some basic trip planning and highlight a few worthwhile places to visit.

 

 

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