What Is An Esker Anyway?
Eskers are only found in areas that were once glaciated. Because
they were created by glacial meltwater they tend to meander across
the surface and may resemble the drainage patterns of rivers and
streams of today. In North America most eskers are located south of
72º N, suggesting that eskers require a wet-based glacier for their
formation.
Eskers are found in southern and western Canada; however, they are
found in the greatest quantity in the north. The largest eskers can be
found in northern Quebec, Victoria Island, and the western Canadian
Shield. Drumlins, spillways, kames, moraines, outwash plains and
clay plains are common in many of the areas where eskers are found.
There are three commonly held views on where esker deposits form within a glacier:
1) in englacial or subglacial channels in the form of tunnels. If the tunnel is englacial, then it is
possible that sediment was lowered to the surface upon melting,
2) in ice-walled trenches at the base of the glacier or on the surface, or
3) they are formed at the terminus as fans or, if the ice margin is underwater, as deltas.
Glaciofluvial deposits differ very little from
regular fluvial deposits. Eskers usually show
beds of sand and gravel in varying
thicknesses, largely dependant on the nature
of the meltwater flow. Cobbles and boulders
are common where powerful meltwater
regimes existed while fine grained laminae
may be found in areas where water stagnated
or low discharges were present. Parallel and
cross-bedded lamination can both be found
within an esker environment.
Drewry (1986) has defined an esker as "a sinuous, narrow, steepsided ridge composed of irregular stratified sediment deposited in
contact with (glacial) ice in either an open channel or an enclosed
conduit." Eskers have been found to be straight in most instances;
however, some do tend to meander. Eskers have been found with
either flat tops or sharp crests.
Although esker dimensions differ greatly, the most common eskers
are in the order of 50 metres high with some achieving heights of
80 metres. Widths are usually less than 150 metres. The lengths
of eskers deviate greatly with smaller deposits being about 100
metres long while the largest eskers reach hundreds of kilometres
in length.
Esker Explanation: Southwestern Nova Scotia (
http://www.gov.ns.ca/natr/meb/field/vista2.htm )
We are flying over a long esker system southeast of Fifth Lake Flowage and west of
Kejimikujik Lake (see stop map). An esker is a sinuous ridge made up of sand and gravel,
formed in a meltwater tunnel under a glacier. Can you spot the ridge snaking across the
landscape? These were called "Indian Ridges" by early European settlers of North America
who thought they were man-made. These ridges trend eastward, indicating a flow of water
from a glacier sitting over Kejimikujik Lake at the end of the last glaciation (see Story of
Glaciers in Maritime Canada).
As we zoom in for a closer look, the ridge clearly stands out against the boggy areas beside
it. There is a parallel gravel ridge probably representing the confluence of a couple of
subglacial tunnels under the former ice cap. This ridge might be a good place to have a bite
to eat. A little wine and smoked salmon perhaps?
We're almost ready to land and the ridge is clearly seen. There is a path along the
esker, used not by people, but by the local moose and deer populations. It is
obviously much easier to walk along the esker (a glacially constructed natural
road) than through the surrounding bogs.
While we're having lunch I will dig up a bit of the ridge with my plastic lunch spoon to
look at it's internal makeup. What we see inside the esker is sand and gravel sorted by the
action of fast-flowing glacier meltwater. The dip of mega-ripple cross-beds tells us the
meltwater flow direction. In this case towards the east. Glacier hydraulics are controlled by
the thickness and temperature of glaciers. As you might expect, these eskers formed during
the melting stages of glaciation between 13 and 15 thousand (radiocarbon) years ago.
Above is a cartoon explaining the formation of eskers and other glacial landforms. A
valley glacier collects material from rivers of glacial meltwater on top of the ice, adjacent
to the glacier, and in tunnels underneath the ice. Sometimes ephemeral lakes form
between the valley glacier and the surrounding hillsides. This would be a the case in the
Parrsboro and Margaree valleys of Nova Scotia, for example. Toilet-bowel like
depressions called "moulins" form on top of the glacier, where meltwater funnels down to
the base of the ice. When the ice melts, waterlain material (sand and gravel) deposited in
the lakes beside the ice and later slumped down on the valley sides are called kame
terraces (B). Material melted out underneath the ice is called glacial till (C). Sinuous
ridges of gravel and sand (eskers) form in tunnels under the ice (D). Hills of gravel and
sand formed when the toilet bowls (moulins) melt are called "kames" (E). Circular holes
in the kame terraces form when ice blocks left in the sediment melt. These are called
"kettles" (F).
Assignment
You receive the following e-mail from a friend:
Okay Mr. Jones wants us to explain what an esker is. I sort of understand a bit from that
part in Chapter 15 but I still don’t really understand what they are…and he is going to
give us that comp. quiz soon and says that there’ll be stuff about eskars in it. This is
English class, not geography!! Could you do me a massive BEAST of a favour and fill
me in a bit.
Writing A Letter Template
Organizing Information By Importance
What is being discussed? (Defining Your Topic)
What is an esker? Where do eskers come from originally? How big are they? How
high, wide? What are they made of and how were they originally formed?
Your Findings (Main Points)
1.
2.
3.
4.
5.
6.
7.
The Letter:
Hey ______________________:
(Greeting)
“Never argue with a man who’s asking for a BEAST of a favour…that’s what I’ve
always said. So you want to know about eskers huh. Well you’ve come to the
right guy…
(Overview: What are you going to be talking about?)
(Main Body: Provide all of the details that you know using order of importance.)