, 9 tweets, 4 min read Read on Twitter
Once upon a time, an underwater landslide (turbidity current) buried this particular patch of sea floor.

The landslide deposits (turbidite) is that tall yellow sandy bit.
Turbidity currents are an interesting style of landslide because they’re density-driven flows: a dense fluid (water & sediment) running out under a less-dense fluid (seawater) across solid ground.

They can be fast or slow.

More: gizmodo.com/when-freshwate…
Stratigraphy is how geoscientists learn the order of events for a landscape. I always think of it like unravelling a murder mystery where your crime scene is rock.

It only has 6 rules, but you won’t need all of them to read these particular rocks.
1. Everything starts flat.

The pencil indicates we’re looking straight down at the ground, edge-on to history.

@ZaneJobe’s caption tells us “up” is right. Same rocks as in QT, rotated with an UP arrow. The outcrop is mostly thin flat layers of a darker brown-yellow rock, but it has a thick raised yellow layer from the left that extends halfway before pinching out. Another solid raised bed crosses completely at the top, but it’s boring.
2. Big areas of similar rocks in the same layer are probably related, even if they’re interrupted.

This is a small area (thanks, pencil!) with no interruptions, so we can skip this one.
3. Old stuff is on bottom, young stuff is on top.

The ground (green line) needs to exist before a landslide can run across it. Same as before, but YOUNG at top, OLD at bottom, and a green line running under the yellow pinched-out deposit. That deposit is the turbidite.
Don’t need the rest:
4. If something happens to a rock, the rock must’ve existed before it happened. We’re big on establishing rocks can’t time-travel.

5. If a rock has pieces of another rock, the pieces come from an older rock.

6. Found fossils? That’s how old your rock is.
Now it’s your turn:
Look at how the rock is deformed around the turbidite pinchout (where the landslide ends).

Notice it’s not-flat in different ways on either side.

Use Rules 1 & 3 to explain how you could tell which way us up if we hadn’t told you. Same photo as before, but with the pinch magnified.
Hint:
Imagine a sandy beach at low tide, or how a lake bottom feels when you wade out. Is it hard or soft? Does it squish between your feet?

If you sand still in the surf line with wee waves crashing against your shins, does sand move above &/or below your feet? How?
Missing some Tweet in this thread?
You can try to force a refresh.

Like this thread? Get email updates or save it to PDF!

Subscribe to Mika McKinnon
Profile picture

Get real-time email alerts when new unrolls are available from this author!

This content may be removed anytime!

Twitter may remove this content at anytime, convert it as a PDF, save and print for later use!

Try unrolling a thread yourself!

how to unroll video

1) Follow Thread Reader App on Twitter so you can easily mention us!

2) Go to a Twitter thread (series of Tweets by the same owner) and mention us with a keyword "unroll" @threadreaderapp unroll

You can practice here first or read more on our help page!

Follow Us on Twitter!

Did Thread Reader help you today?

Support us! We are indie developers!


This site is made by just three indie developers on a laptop doing marketing, support and development! Read more about the story.

Become a Premium Member ($3.00/month or $30.00/year) and get exclusive features!

Become Premium

Too expensive? Make a small donation by buying us coffee ($5) or help with server cost ($10)

Donate via Paypal Become our Patreon

Thank you for your support!