Archive for the ‘Basin and Range’ Tag

Mountain Monday: Outback Oregon   4 comments

Winter begins by dusting the Pueblo Mountains of southeastern Oregon.

Winter begins by dusting the Pueblo Mountains of southeastern Oregon.

I posted Friday on photography around stormy weather but neglected to include snow.  Good images are really difficult to get when it’s snowing heavily.  So let’s follow up and correct that error.  This is an image where the snow had just fallen on the mountains but never really reached me.  It was early morning and I was hoping for the mountains to show themselves.  It was chilly so I though maybe there would be snow, but I was surprised there was so much.

I was in what is called Oregon’s “outback” (apologies to Australia).  Southeastern Oregon is very thinly populated and is wide-open high desert.  Geologically, the mountains are fault-block type.  This simply means that they were formed by high-angle faults which throw one side down (becoming the valley or basin) and one side up (forming a long relatively narrow range).  It’s also known as basin and range terrain and continues south through most of Nevada and east to the Wasatch Mountains of Utah.

The reason I didn’t get snowed on is because of the “rain shadow effect”.  This is when rain or snow is essentially blocked by a mountain range.  The clouds are lifted by the mountain slopes, cooling the air and causing precipitation.  When the air descends the lee side of the range, it warms and dries, leaving little or none of the wet stuff for the valley beyond.  In areas where the weather pretty much comes from one direction, there can be very dramatic differences in vegetation between the windward and lee sides of any range that runs nearly perpendicular to the direction of prevailing winds.

Enjoy your week and Happy Labor Day to my fellow Americans!

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.

Death Valley IV: Geologic Features   Leave a comment

This is the first of three posts on the geology and ecology of Death Valley National Park in California.  Death Valley is Disney Land for geologists, and for anybody interested in earth science.  What isn’t as well appreciated is it’s also a very special place for desert ecologists and botanists.  But first the geology:

A colorful dawn breaks over Death Valley National Park in California.

A colorful dawn breaks over Death Valley National Park in California.

Since it is the driest place in North America, vegetation does not cover geologic features at Death Valley.  And since it lies in a place where there’s been a lot of geological action for an awfully long time, there exist a great variety of rock types and structures.  Regarding the latter, the whole region has been first smashed by mountain building and more recently torn apart by rifting.  Death Valley’s structure (meaning twisted and folded rocks, fault zones, etc.) shows this in dramatic fashion and is one of the major draws for geo-types.

I first visited Death Valley with my first year geology class.  We came down on Spring Break from drippy Oregon and boy was it nice to be in warm sunshine for a week.  We all got 3 credits for it, but it was a lark!  Since my professor was a biologist and avid birder as well as a geologist, he mixed ecology and raptor-spotting in with rocks for a really complete picture of this amazing place.

The soaring dunes at Mesquite Flat in Death Valley National Park, California.

The soaring dunes at Mesquite Flat in Death Valley National Park, California.

GEOGRAPHY AND CLIMATE

Death Valley is an enormous trench.  The vertical relief from Badwater at -283 feet elevation to the top of Telescope Peak is about 11,300 feet (almost 3500 meters)!  This giant steep-walled valley is called by geologists a graben (German for grave).  Steep fault zones, called “normal” faults, force the bordering mountain ranges up while the valley drops and fills with sediments.  This sort of faulting is repeated across the Basin and Range Province of Nevada and bordering states.

The steep mountains left by the normal faults to stand high above valley floors block moisture coming in from the Pacific and cause an extreme form of the “rainshadow effect”.  The Sierra Mountain Range, which tops out at over 14,000 feet at Mount Whitney, gets most of the rain and snow.  The Panamint Range, which borders Death Valley to the west, also gets its share.  This leaves almost no moisture for Death Valley.  That is why years can pass without any rainfall.  It is extremely arid, and this of course causes the plant and animal life to be sparse.  But the fascinating adaptations that have evolved in the life forms at Death Valley more than makes up for the paucity of biomass.

Basin and Range structure has led to two types of features.  These features, both of which are displayed at Death Valley, determine much of what goes on geographically, ecologically and even with human history here.

The extensive salt flats near Badwater in Death Valley National Park, California.

The extensive salt flats near Badwater in Death Valley National Park, California.

PLAYAS

 First thing you’ll notice are the playas (or pans), which are dried up lake beds.  These flat surfaces, which can be floored in white salts or a tan clay surface, are caused by internal drainage.  Because of the normal faulting described above, water that washes from the ranges into the basins of the Basin and Range often never makes it out along a river course. Instead, the water collects in large, shallow lakes.

When the water evaporates, salts (chlorides and sulfates of sodium, calcium, phosphorous, etc.) are left behind in the lakes.  These so-called evaporites are too heavy to be lifted into the air with the water vapor.  (This is why rainwater is fresh and why the oceans are salty.)  The salts come from weathering of the minerals in rocks of the surrounding mountains.

The full moon sets just as morning light hits the cracked salt flats near Badwater, North America's lowest point, in Death Valley, California.

The full moon sets just as morning light hits the cracked salt flats near Badwater, North America’s lowest point, in Death Valley, California.

The evaporite minerals are inevitably concentrated into the shrinking pools of water, where they crystallize into fascinating patterns.  This happens during most seasons (winters are wet and summers very dry), and so salt layers build up.  Gypsum and borax are also formed in this way.  Death Valley’s human history includes the charismatic 20 Mule Team borax story.  Near Badwater in Death Valley proper, a huge salt pan is spectacularly developed.  Take the West Side road for the best access.

 Go over to Panamint Valley in the western part of the park to see and walk on a great playa.  It was formed when fine sediment was deposited instead of pure salt.  Certainly Death Valley’s best-known example of this is Racetrack Playa, where stones appear to have skated across the playa, leaving behind their tracks.  It’s still uncertain how they move, but winds and a thin layer of ice probably have something to do with it.  Note that to visit the Racetrack in the far northern part of the park requires driving a long, long washboard gravel road.  And to make things worse, the road bed is made of especially sharp gravel, so you’ll need very good tires (and two spares).

A close view of the ridges that form the salt polygons at the Badwater salt flats, Death Valley N.P., CA.

A close view of the ridges that form the salt polygons at the Badwater salt flats, Death Valley N.P., CA.

ALLUVIAL FANS

But mostly what you’ll see in Death Valley are the other feature that result from Basin and Range faulting.  As you drive through the park, one thing you’ll notice is that this is a rocky desert, not so much a sandy one.  As you look across the valley, you’ll notice large semi-circular (fan-shaped) gravel features that narrow to a point at the canyon mouths.  These are alluvial fans, and they form everywhere that rapid uplift of mountains overwhelms the ability of rivers to transport the debris out of there.

Try walking up an alluvial fan and you will get a feel for their deceptive steepness and difficult, loose surface of cobbles.  But it’s a great education on how they form.  You’ll also see desert varnish, a dark, sort of rust that forms on the rocks when they sit undisturbed for a long time.  I rarely link to Wikipedia, but heck, go ahead and check out desert varnish.  It’s an  interesting, part living feature of the Mojave.

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.

When alluvial fans merge into a wedge of debris that flanks the entire range of mountains, it is called a bajada.  Eventually the mountains disappear and all that’s left is a gravel plain.  Namibia has extensive ancient gravel plains, but the American West is really much younger.  Large outcrops that stick up island-like out of alluvial fans or bajadas are called inselbergs.  Great words in geology!

I’ll get to the “rest of the story” in my next post.  I miss Paul Harvey!

The pre-dawn hours in Death Valley's sand dunes promises a beautiful sunrise.

The pre-dawn hours in Death Valley’s sand dunes promises a beautiful sunrise.

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