WEBVTT

00:00:00.000 --> 00:00:02.168
Which direction is up?

00:00:02.168 --> 00:00:05.964
This might seem like a silly question,
but the answer isn't always obvious.

00:00:06.548 --> 00:00:10.427
Rock layers can be tilted so much
that the older layers are on top.

00:00:11.219 --> 00:00:14.222
Geologists call these overturned beds.

00:00:14.347 --> 00:00:17.976
Luckily, geologists
and paleontologists have several tools

00:00:17.976 --> 00:00:20.979
to figure out
if layered rock has been overturned.

00:00:21.229 --> 00:00:24.232
One tool is footprints and track fossils.

00:00:24.274 --> 00:00:27.027
The top side of these fossils
will look like a depression,

00:00:27.027 --> 00:00:29.654
while the bottom side
will look like raised bumps.

00:00:29.654 --> 00:00:33.491
The cracks that form when mud dries out
often get preserved in the rock record.

00:00:33.533 --> 00:00:35.618
When we find mud cracks in old rocks.

00:00:35.618 --> 00:00:38.079
We know that the cracks open downwards.

00:00:38.079 --> 00:00:40.832
When ice melts over a lake or ocean
and drops

00:00:40.832 --> 00:00:44.961
a large rock onto a layer of soft silts,
this is called a dropstone.

00:00:45.045 --> 00:00:48.423
The layers underneath
the falling stone get bent and deform.

00:00:48.423 --> 00:00:49.674
As the rock gets buried
the layers on top remain flat.

00:00:51.593 --> 00:00:54.596
Root fossils are one of the best ways
to find up.

00:00:54.846 --> 00:00:57.932
Roots always grow from the top
and spread downwards.

00:00:58.266 --> 00:01:00.602
Another tool is graded bedding.

00:01:00.602 --> 00:01:03.772
This forms when mixed sediments
fall rapidly into water.

00:01:03.938 --> 00:01:07.233
The larger sediments settle out first
and the finer grained sediments

00:01:07.233 --> 00:01:08.693
cover them over time.

00:01:08.693 --> 00:01:11.613
This leads to a finer
grain size towards the top.