Practice English Speaking&Listening with: Yosemite Nature Notes - 10 - Rock Fall

Difficulty: 0


This is actually a really interesting viewpoint

to view the runout zones for rockfalls from Glacier Point.

Usually we're looking either across the Valley

over at Glacier Point,

or we're down at the bottom looking up,

and those are valuable views as well,

but this is really useful to be able to be up here

and look down.

And from right here I can see runout zones

for about five different rockfall events

that have occurred in the past decade or so.

The cliffs are eroding very slowly, grain by grain,

all the time, but when suddenly a piece of the cliff

that's 200 feet wide and 100 feet tall

and 20 feet thick just falls off the cliff

and falls 1,000 feet to the Valley floor

and breaks up in all these pieces,

you know, that's a really different style of erosion.

When we look around Yosemite Valley,

we see abundant evidence that rockfalls have been occurring

here for thousands of years.

Yosemite Valley is a glacially carved canyon,

and since glaciers retreated about 15,000 years ago,

rockfall has been the major force

shaping this landscape.

We don't always know what causes a rockfall

in a particular event.

Sometimes it's obvious in that

there might have been a large rainstorm or snowstorm

that then caused a lot of seepage and so we surmised that,

that seepage must have caused the rockfall.

It's not always the case, sometimes

rockfalls happen without any known trigger,

so it could just happen on a nice bright summer, sunny day.

We had decided to rent bikes to ride around the Valley,

we were only here about an hour and

heard this loud thunderous roar and

couldn't tell where the sound was coming from.

And looked around and I saw a rock falling off the wall

over to my right.

Whoa, a rockfall, look!

I had to get my camera off the bike as quickly as I could,

that's why the footage is a little shaky.

I had to take the wide angle lens off of it

and was lucky enough to catch the third rockfall.

Look at that piece!

In the particular rockfall that was captured on video,

you can see a rock slab detaching from the cliff face,

sort of skipping down the cliff face,

hitting a prominent ledge,

breaking up into a bunch of pieces.

Some of those individual pieces are 40, 50 feet on a side,

and those pieces then are free falling through the air,

spinning on their way down, they fall several 100 feet,

and then they impact the talus slope at the bottom.

And some of those huge boulders will be moving

down the talus slope at 40, 50 miles an hour,

snapping large trees like matchsticks

and going all the way down to the base of the talus slope

and the Valley floor.

We're standing at the base of the rockfall that happened on

August 26 of last year, 2009.

And this is one of the rocks that fell from it.

I thought they were about that big, you know,

because we were about a half mile away,

so they didn't seem that big.

But now looking at it, they're huge.

It's amazing to think that it came down from up there

and then bounced, I guess, it must have bounced, right?

And prior to August of 2009,

this slope had large oak trees on it,

but when several thousand tons of rock

came down off the cliff

and landed in this area

and then moved down the talus slope,

those boulders basically wiped out

all of the trees that were on this slope.

And this talus slope leads right down to the floor

of Yosemite Valley

and just beyond the edge of the talus slope

is the Ahwahnee Hotel.

If these rockfalls were occurring in a remote valley,

they would be of scientific interest only,

but because these rockfalls are occurring in Yosemite Valley,

a narrow Valley with nearly four million visitors a year,

they are more than a spectacular natural process.

Depending on where and when a rockfall occurs,

it can have potentially serious consequences.

In 1971 and 1972, there were two large rockfalls

that came off the face of Elephant Rock.

This was the impact area for the March 1987

Middle Brother Rockfall.

This is the impact area for the July 10, 1996

Happy Isles Rockfall.

This boulder here that I'm standing on,

roughly a 400-ton boulder

that came down in October of 2008.

So this boulder here is an old rockfall boulder,

and you can tell that

this is an old rockfall boulder because

it's covered in lichen and moss,

it's not a fresh boulder.

Now, in contrast, this boulder here is fresh,

it has no moss or lichen growing on it.

And this boulder is just like those other boulders

that came down from earlier rockfalls,

the difference is, when this boulder came down

in October of 2008,

these structures were here in Curry Village

and the consequences of that are obvious.

A scary situation hitting visitors at Yosemite today,

for the second time in as many days,

a rockslide has hit the Park's Curry Village area.

We are getting some eyewitness accounts of that rockslide

that hit this morning, sending people running

and crying in fear,

and there are some unconfirmed reports

of injuries in that area.

Now, we have some pictures from KCRA 3,

LiveCopter 3 HD was over the scene of that rockslide

this morning that took place at about 7 o'clock.

Our primary concern is saving people's lives.

15 people have died from rockfalls

in Yosemite National Park in the last 150 years,

and that's not an insignificant number,

it is much smaller than the number of people

that have died in Yosemite streams and rivers,

and it's certainly much smaller

than the number of people that have died in traffic accidents.

But one big rockfall at the wrong place

the wrong time could dramatically increase

that number and that's why we are focusing on

learning everything we can about rockfalls,

using laser scanning, computer modeling,

monitoring of the cliffs.

But the scope of trying to understand

all of these complexities

and all of these different rock faces,

to a point where we can start to predict rockfalls,

is going to be really challenging.

Predicting rockfalls is a very difficult scientific question

to go after, it's similar to predicting earthquakes.

Rockfalls can happen anytime

and so we're interested in determining

how often and when and potentially where.

So much of what we know of the Yosemite landscape;

the iconic cliffs of El Capitan and Half Dome,

the forest at talus slopes,

the big boulders out in the floor of the Valley,

all these features owe their existence to rockfalls,

rockfalls have been occurring in Yosemite for thousands of years

and they will continue for thousands more.

The Description of Yosemite Nature Notes - 10 - Rock Fall