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:
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.
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.
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 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).
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.
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!