Wednesday, May 2, 2012

Strangers in a Strange Land: Just how flat is flat?

We've looked at two playa surfaces now in the series Strangers in a Strange Land, our tour of Death Valley and the Mojave Desert of eastern California. The first was the salt pan at Badwater, the lowest point in North America at minus 282 feet, and Bonnie Claire Playa, where rocks seem to have a mind of their own.

Playas have to be counted as pretty much the flattest places on Earth (abyssal plains on the deep ocean floor are pretty flat, too, but aren't exactly accessible). And they are flat. When rare flash floods bring clay-rich water onto the playa surface, the water spreads out, and the clay stays in suspension long enough to be spread evenly across the lake.
I was thinking about flatness today when I was running errands at the department. Topographic maps are the tool of choice for geologists for mapping and plotting. They are also used by anyone who really wants to know where they are when backpacking, hiking, camping, hunting, or whatever. I generally blame topo maps for dragging me into geology. I was a scout, and I was fascinated by the maps we used while hiking. I collected them, I practiced my orienteering/compass skills constantly, and eventually became the "go-to" guy when we were out in the wilds. They called me Captain Eagle-Eye Wrong-Way, because it was just about impossible to get me lost if I could see any two mountains and had a compass and map (even today students ask "How far?" and when I answer, they say "Garry-miles or real miles"?).
A couple of years back I was going through the department map collections, and I found a map that I realized would defeat my efforts at orienteering. I saluted the map-makers, and posted the map on the wall in the department to be a shrine to the pratfalls of topographical arrogance. It is pictured below, and people familiar with contour lines will notice a near complete lack of them. The red lines form a grid made of square miles, so this map is about 6 1/2 by 8 miles, or about 52 square miles. The contour interval is 5 feet, which means that elevations across this entire quadrangle vary by only a few feet, primarily because of some 8 foot high dunes on the lower left-hand corner.
The map is part of the Carson Sink, a place in western Nevada where a number of rivers come to die. This was part of the ghastly last 40 miles where settlers in the 1840-50s came to grief as they tried to reach the California gold fields. Even today, a notation on the map mentions "no roads or trails". Well played, topographers, well played!

4 comments:

Rebecca Gonzales-Clayton said...

Now that's cool!

Karen said...

What fascinates me most in the desert isn't the flats, but the deceptively gradual alluvial fans. They look fairly flat (though that's in relation to the mountains behind them) yet have a fair bit of relief, and hiking up a fan gives one a workout!

Garry Hayes said...

No kidding, Karen! We used to run our cross country practices in high school on the alluvial fans above Ontario. Deceptively and exceedingly steep!

Karen said...

A note on playas, though: my in-laws traveled across Utah's Great Salt Desert just after some heavy rains. They stopped at a rest area, Mom stepped off the surfaced area into the salt pan -- and of course, sank into the mud. "Oh, you've waded Lake Bonneville!" I said. For some reason she didn't think it was funny.