Once upon a time, about  27,000 years ago, ice sheets stretched across a large chunk  of the Northern Hemisphere.

They covered Canada, extended down  into the United States, encased Iceland completely, and blanketed  Scandinavia and the British Isles. Some of that ice still  covers Greenland to this day.

Everything the ice touched, it changed. These glaciers literally carved  the geology of these regions. And although many of those glaciers are now gone, they are still shaping the landscape, in some ways you might not expect.

Here are five ways glaciers and  are still changing the world. That’s the kind of niche top 5  countdown you can only get on scishow! [♪INTRO] Let’s start with some stunning views  you might already be familiar with: the fjords of Norway. Fords are a brand of American automobiles, with the Mustang being an iconic- Oh sorry.

Fjords are long, narrow channels  of water bordered by towering, near-vertical cliffs. These  steep, U-shaped structures didn’t develop overnight. They probably started as v-shaped valleys, carved by rivers  over the course of millennia.

But rivers aren’t the only thing that can flow, glaciers can too, like rivers of ice. So when they started to flow from  their mountain peaks downslope toward the sea, they would have  followed the path of least resistance. Such as a convenient, pre-carved river valley.

Glaciers are mostly made of ice, but as they flow, they also pick up some pretty big rocks and carry them along for the ride. You know how some birds swallow  rocks to help grind up their food? Glaciers kind of do the same thing.

The rocks they “swallow” are  pressed against the ground by the weight of all that ice, chewing up  the landscape as the glacier moves. Over tens of thousands of years, a glacier gouges a river valley into a fjord,   and can carve the ground even after  the glacier pushes out to sea! These glaciers were massive pieces of ice, heavy enough to sink and drag along the sea floor for a while before forming  a floating glacial tongue.

Which is the actual term for the bit that sticks out off land and into the sea. This eating metaphor is really rock solid. When the glaciers eventually vanished, their water didn’t just disappear,  it their water melted into the sea.

The fjords’ U-shaped bottoms,  where the glacier used to rest, were then further submerged. As a result, most fjords  plunge deep below sea level, with only their characteristically tall  cliff sides surging above the water. And the valleys that are left behind form a map of the glaciers’  slow-motion, downhill treks.

Even though the ice melted, all those rocks the glacier uprooted had to go somewhere. They became part of the glacier,  flowed with it, and, when the glacier melted, were often deposited  far from their places of origin. That’s exactly what shaped  so much of our next example, the American Midwest.

When you think of the Midwest, you  might picture unending flatness, with fields of corn as far as the eye can see. Having driven through Ohio before,  it's almost unsettlingly flat. And glaciers have a lot to do with that flatness, because they chewed up the ground  as they spread across the land, similar to how they carved all those fjords.

And because they stretched so far and wide, this action smoothed and eroded  every place the ice went. At one point, the Laurentide Ice Sheet blanketed the American Midwest, pushing it flat. When it eventually retreated,  it left behind a lot of the sediment it picked up along the way, creating a layer of soil that’s more than 30 meters deep in some places.

But that soil, called glacial  till, isn’t just flat. Research suggests that glacial  till soil is also more fertile. Which could help explain why the Midwest became America’s breadbasket.

Glacial till is rich in nutrients  like phosphorus and potassium, and micronutrients like zinc and selenium. Experiments have shown that growing in glacial till can improve crop yields. So some researchers are even  considering putting it in fertilizer.

And while we’re still figuring out exactly how nutritious glacial till  is and why it got that way, some people argue that the  Midwest’s glacial past is exactly what shaped it into the  agricultural powerhouse it is today. But flatness and productivity aren’t the only Midwestern features with glacial origins. Glaciers also formed many of the  Midwest’s distinct landmarks, like moraines, which are big piles of stone that were deposited at the edges of the glacier.

You can spot moraines around Lake Michigan, like the Valparaiso Moraine. Or there’s the Itasca Moraine in Minnesota. Glaciers can also form drumlins: elongated, egg- or oval-shaped hills that all face the same direction like the scales of a dragon.

We still don’t fully understand  how drumlins are formed, but our best guess is that they’re  formed close to the edge of the ice sheet, and that they’re  all facing the same direction because they’re aligned with the  direction of the glacier’s flow. Horicon Marsh, Wisconsin has one of the highest concentrations of drumlins in the world. Another fun type of deposition are erratics, large rocks that get their  name because they’re chemically non-native to the environment they’re found in.

Basically, the glacier  picked them up at some point, carried them a long way, then  dropped them off somewhere completely new, like an  extremely slow alien abduction. They’re common in the Midwest,  like Waldo’s Rock in Iowa! This massive boulder matches  the composition of rocks that are hundreds of kilometers  away in central Minnesota.

And glaciers didn’t just shape the  scenery around the Great Lakes. They formed the Great Lakes themselves. But before we get to that,  all science needs funding, so let’s go to a quick break.

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You can upgrade to Premium to unlock all courses. And right now, SciShow viewers can save 20% off an annual subscription at brilliant.org/scishow. Now when the glaciers that  formed the fjords retreated, all that meltwater flowed into the ocean.

But sometimes a glacier melts in the middle of a continent and that water has nowhere to go. In those cases, the water ends  up caught in basins carved by the glacier itself. Voila!

You’re  now the proud owner of a Lake! Sometimes, we can even  determine a glacier’s path based on the lakes it left behind.  For example, paternoster lakes. Paternoster lakes form in a  chain, separated by moraines.

They follow the path a glacier takes when it retreats like Hansel and  Gretel’s bread crumb trail. Essentially, the glacier takes a big step back, depositing stones that form a moraine. It then takes another step  back, forming a second moraine.

Meltwater gets caught in between those two. Then it takes another step  back, and so on, and so on. When you look at them from the air, they look like a string of beads,  and actually get their name from their resemblance to beads on a rosary.

But glacial retreat doesn’t  just form cute, little lakes. There are enormous glacial lakes, too. The biggest of which are the Great Lakes, a collection of five lakes between  the United States and Canada that together hold a fifth of  all the freshwater in the world.

Remember the Laurentide Ice  Sheet that flattened the Midwest? Well it was enormous. In some places, the ice was three kilometers thick or more.

And in its heyday, the Laurentide covered everything from the North Pole to Pennsylvania. The sheer weight of all that  ice carved out the basins that eventually flooded with meltwater  and became the Great Lakes. The boundaries of the lakes  were then firmed up by moraines, which were deposited by the glacier as it melted.

A few millenia later, those boundaries were further shaped by another glacial phenomenon. While the Great Lakes’ basins were  carved out of the earth by the Laurentide Ice Sheet, the lake  boundaries reached their final forms thanks to a phenomenon called  glacial isostatic rebound. Those are some big geology words to describe a pretty simple process.

Albeit one that is happening on a planetary scale. Okay, picture a trampoline. When you sit on it, your  weight makes the material flex, and you end up dropping a little bit.

When you leave, the trampoline  springs back into place. Earth might seem pretty rock solid, but like a trampoline, it’s actually elastic. When enough force is applied, it bends!

You just need to apply a lot  more force than you’d need for a trampoline, and the  bending happens a lot slower. Glaciers as big as the  Laurentide Ice Sheet were heavy. The weight of all that ice squished the surface of the earth into the mantle.

And then when the glacier melted, the ground breathed a sigh of relief  and started to spring back up. While the entire Great Lakes  region is constantly rising, isostatic rebound helped  shape the lake boundaries, pushing everything into the  shorelines we know and love today. And believe it or not, but this  glacial rebound is still happening!

For example, Ontario, Canada, used to be right underneath the glacier, right  in the middle of the trampoline. That means parts of Ontario  are getting taller every year, by a whopping 13 mm. That’s  about half an inch a year, which is really fast for a geological  process, and definitely noticeable.

I mean, if I were still growing half  an inch a year I wouldn’t have had to   abandon my dreams of being a basketball  star to become a science communicator… Uh, anyway But what goes down must come up, and vice versa. While some parts of North  America that were weighed down by ice are still getting taller,  other areas are still sinking. These sinking areas were  once part of the forebulge, the area that got squished out at  the leading edge of the glacier.

The forebulge was pushed upward by the weight of the glacier behind it,  and now that the glacier is gone, is still relaxing back down. These sinking regions are mostly along the coasts, which certainly doesn’t help with  the issue of rising sea levels. And this same slow motion trampoline settling is  also happening for our friends across the pond.

Most of Scotland is getting a wee  bit taller each year, while much of Ireland and England are slowly  slouching down into the sea. So far we’ve looked at geographical features formed by glaciers grinding,  squishing, or melting. But what happens when a region seems to suspiciously lack any glacial features?

There are some really unique landscapes that came about because glaciers weren’t there. Nestled in the glacier-smoothed  Midwest is a spot around Wisconsin that the glaciers  somehow just missed entirely. This region is called the Driftless Area.

It’s a pocket of steep, rocky  bluffs and deep, winding valleys. A hikers’ paradise in the  flatness of middle America! The Driftless Area gets its name  because it’s missing the drift, the sand, silt, and debris kicked up by glaciers, And it escaped that deep freeze  thanks to its topography.

The Driftless Area is a high elevation zone bracketed by lower-elevation areas that turned into Lake Superior and Lake Michigan. The region stuck up above  the rest of the landscape enough that the ice sheet flowed mostly around it. Consequently, it was never eroded flat.

Then there’s a missing river in Washington State’s Channeled Scablands; a region of exposed bedrock marked  by suspiciously dry channels. It sure looks like a place  where water should be flowing, and there are a couple rivers around there. But none of them are anywhere big enough to have carved the Channeled Scablands landscape.

Water did flow there once, but it  disappeared nearly 20,000 years ago. The Scablands formed when a dam  made by a nearby glacier broke. That glacier happened to be propping up an enormous lake called Glacial Lake Missoula, so when the dam broke, all that  water came flooding out at once.

Over 2000 cubic kilometers of water ran over what is now Washington State. It was a series of floods so  catastrophic that they sometimes reshaped the landscape completely  in just a matter of days. These floods happened many  times over several millennia, repeatedly re-etching the land, before drying up and leaving the Scablands behind for us to ponder.

There are so many other cool glacial features that we couldn’t quite fit in this episode. Like Mount Sunday in New Zealand, which some of you might recognize as Edoras, the capital city of Rohan from  the Lord of the Rings movies. Mount Sunday is what’s  called a sheepback mountain, and was carved by glaciers, once upon a time.

Glaciers also form beautiful  structures like mountain cirques, deep basins that make them look like volcanoes. Not to mention braided river  systems and glacial meltwater deposits called eskers, among  other stunning natural features. As we’ve seen, the world is still reshaping after the melting of our ice sheets.

New cases of isostatic  rebound are also springing up, especially in Greenland, as our remaining ice sheets melt due to climate change. Although many of the Earth’s glaciers have been gone for tens of thousands of years, they left us distinctive  geographical features that shaped the landscape and the ways we  navigated and settled the planet. Even in their absence, we  still see glaciers everywhere. [♪OUTRO]