Geology Reference
In-Depth Information
perched well above the modern river. Others were lake terraces made of layered silt and
clay. How did they get there?
A coherent picture began to emerge as we traversed up and down the valley. The terraces
made of river gravel continued down the valley bottom to an elevation corresponding to the
top of the lake sediments, defining the ancient shoreline where the rivers had entered the
lake. In addition to prominent terraces that rose a few hundred feet above the modern river,
the remnants of a second set of higher terraces preserved at a few remote locations halfway
up the valley walls attested to an even deeper lake. At least twice in the recent geologic
past a lake extended hundreds of kilometers upstream from the Tsangpo Gorge. I was onto
something.
It was thrilling to have scientific sleuthing that started as little more than a hunch lead
to a solid story. Once I saw the pieces and knew how they fit together, the story of ancient
lakes that once filled the valley of the Tsangpo stood out plain as day in the form of the
land.
What do you see when you look at the land? Something stable and reassuringly solid. A
slope to ski down? A surface to pave over? Geologists see the world as incredibly dynamic
and ever-changing—only change occurs slowly over immense spans of time.
I've learned to see what the land used to look like, and what it might look like in the fu-
ture. Reading a landscape is an ongoing process of combining curiosity and inquiry. Why
is that hillside bare and rocky? Why is that one covered with soil? Deciphering topography
makes geologists natural storytellers. We piece together fragmentary clues in rocks and
landforms to connect dots across landscapes, mountain ranges, and continents and tell stor-
ies with whole chapters lost to erosion and time. And here in the valley of the Tsangpo was
a great story, except for one big loose end.
