Glacial Till Features

Speaker: Okay, so this is the difference
between uh, glacial till,

which is unstratified,

and non-glacial till.

So, hopefully you can see
that this, uh, gravel here

is well-stratified.

There are a lot of
sedimentary features here.

Um, you can see graded bedding.

You can also see cross bedding.

Maybe I can just sort of zoom in on that.

So there's cross bedding right there.

Very coarse uh, cross bedding.

However, the stratification
or the horizontal layering,

the general horizontal here,

indicates that this is not glacial gravel.

Even though it's approximately
the correct size

and sorting distribution.

The presence of sedimentary features

as well as, uh, the stratification,

that's the horizontal layering,

indicated as not glacial gravel.

Okay. I'll try and show you
glacial gravel a little later.

So this is uh, glacial till.

It is gravel that is deposited
by a glacier.

Um, and the identifying
characteristics of it

is well-rounded clasts,

however they're not
really really well-rounded.

They are definitely rounded
to some extent.

Um, oftentimes there are striations
on each individual clast,

maybe I can show you that later.

Um, those are scratches that are produced

when gravel bounces or scrapes against

other pieces of gravel or the bedrock.

And the other identifying characteristic

is that the gravel itself
is unstratified.

There is no layering,
no horizontal stratification.

Um, and there really are
no sedimentary features here,

other than the lack
of sedimentary features,

which indicates that
it was deposited by a glacier.

Now hopefully you'll be able to see,

if it'll focus on that,

the scratches
on the surface of this clast.

Those parallel scratches
are glacial striations,

which are present on a clast.

It's stuck in there pretty hard. Argghh.

It won't come out.

And in addition to the lack of uh,
sedimentary features,

um, one of the characteristics
that you always look for

is a very very poor degree of sorting

in terms of clast size.

And so, maybe I can
zoom in on that a little bit.

You've got clasts of all different,
uh, sizes.

Very poorly distributed.

I suppose even down to these sizes,

which have fallen down
from the cliff face, I suppose,

the gravel face anyways.

Very poor sorting.

Okay. Since we're talking about till,

I thought I'd show you
what happens when till, um,

becomes a rock, when it becomes
consolidated or indurated.

This is called tillite, which is uh,

the rock, I suppose, from glacial till,
a conglomerate.

This one is very old.

It is on the side of
the Trans Canada, here. [cars going by]

As you can tell by the traffic.

And it is south of
Wawa in Ontario, Canada.

[traffic in background]

This particular crop has been
highly metamorphosed,

so these in the Canadian shield.

Um, so strongly metamorphosed.

And this tillite is uh,
weathering very rapidly

as a result of the intense metamorphism,

which has caused cracks and fractures

which increase surface area, of course,

and cause weathering to accelerate.

[traffic in background]

Now realize I'm looking at a shadow here,

but hopefully you can see
that the clasts are well-rounded.

Very poorly sorted,
in terms of size distribution.

And are not layered.

They are unstratified.

I just thought you might want to see
what's on the other side of the road.

That is Lake Superior,

about an hour south of Wawa, Ontario.

Pretty big lake, eh?

Kind of looks like the ocean.

You can't see the other side
of Lake Superior,

even though we're way the heck up
on the roadside.

There you can see down to the beach.

Alright, that's it.

Bye for now.