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15 Modern-Day Signs of the Ice Age

Picture this: A massive sheet of ice completely buries Manhattan. It happened 20,000 years ago during one of the Earth’s many ice ages. As heavy glaciers—in some places nearly two miles thick—flowed across the land, they pummeled mountains and scoured rock. Out of this chaos were born some of the world’s most well-known natural features: Loch Ness, Walden Pond, Plymouth Rock, and more.

We’re still in an ice age, technically speaking, though we’re in a warmer period called an interglacial. The glaciers have mostly melted back. But, like bad houseguests, they’ve left a huge mess behind. Here are 15 amazing signs of past glaciation. 

1. THE GREAT LAKES

Ice created the Great Lakes. Here’s how: As an ice sheet flowed along, it carved humongous, deep scars in the bedrock. Once the climate warmed, the melting glaciers filled up the basins with water and sediments. And voilà—giant lakes!

They’re not the only well-known water bodies excavated by glaciers. Flowing ice enlarged stream valleys and made them into New York’s Finger Lakes. It also created one of the most famous lakes of all time …

2. LOCH NESS 

The fabled home of Nessie is also the product of glaciers. When the ice from the most recent glaciation rolled over Scotland, it hit a weak spot in the rock. This vulnerable area was, hundreds of millions of years ago, much like the modern San Andreas Fault, where two plates were rubbing against each other. The glacier picked away at the weak point and dug out Loch Ness and other nearby lochs.

3. WALDEN POND

In 1845, Henry David Thoreau headed into the forest to write his famous book Walden; or, Life in the Woods. The setting was Walden Pond—and it was created by a huge chunk of ice that fell off a glacier.

As the glacier melted, the fallen chunk became buried in dirt and debris. And, as the climate got even toastier, the ice chunk melted, leaving a deep hole full of water. The result was a pond so pristine that it became a symbol of nature.

This kind of water body is called a kettle hole. Sometimes, special wetlands called bogs form in kettle holes. Incidentally, bogs are tough places for plants, since they’re not connected to the nutrient-rich flow of a stream, and some bog plants compensate for the lack of nutrients by eating animal flesh.

4. ANCIENT ROADS

How does a two-mile-thick sheet of ice melt? Enormous streams of meltwater flow through it. And, as in all rivers, these ice-encased waterways carry rocks and other debris. Once the ice sheets melt away, that debris remains in the form of raised gravel beds that snake across the landscape. These are called eskers—and people have been using them as natural roads for centuries.

The ancient Celts crossed Ireland along an esker system called An Slí Mór (Irish for The Great Highway). And you can drive atop an esker on the Denali Highway in Alaska or on Route 9 in Maine.

5. GEIRANGER FJORD

Fjords are some of the ice sheets’ most amazing creations. A fjord is a U-shaped valley that was excavated by a glacier and usually filled with seawater. Norway boasts many of the most spectacular fjords, and in fact, the word fjord comes from Old Norse. One of these natural monuments is the beautiful Geiranger Fjord, a UNESCO World Heritage Site. 

6. RISING GROUND

If you’re standing on a place that was once weighed down by a glacier, then the land below you might be rising. 

Sit in a soft chair and you’ll feel the cushion sink under your weight. When you get back up, it’ll (hopefully) spring back into shape. The land reacts in the same way to the heavy weight of an ice sheet—it squishes down up to a half kilometer (.3 miles), then rises slowly once the ice has melted.

This recovery is so slow that the land is still rising after the last glacial period. And that spells trouble for some people, such as those on the U.S. East Coast. Canada and parts of Greenland—which were weighted down by ice sheets during the end of the last Ice Age—are rising like a seesaw, pushing the East Coast downward. This raises the sea level, which is especially alarming when combined with sea level rise from climate change. 

7. PLYMOUTH ROCK

Legend holds that the pilgrims of the Mayflower landed at Massachusetts’ Plymouth Rock in 1620. It’s an important story in the colonial history of the United States, though the Pilgrims may not have actually landed at that particular rock. But this much is true: Plymouth Rock is in Plymouth because of glaciers.

Glaciers are pretty dirty. As they move along, they pick up dirt and debris, including huge boulders, and dump it elsewhere. Once the glaciers are gone, those large rocks sit alone on the landscape, making people wonder, “How on Earth did this thing get here?” These “things” are called glacial erratics.

Plymouth Rock isn’t the only famous erratic. Pictures of Kummakivi in Finland sometimes go viral with a caption declaring that the rock is a scientific mystery (it’s not). There’s also England’s Merton Stone, Canada’s Okotoks or Big Rock, and many more. 

8. FARMS THAT GROW ROCKS 

Glaciers leave behind smaller stones and gravel, too, in a jumbled mix of rubble called glacial till. That’s why the soil in many previously glaciated places is rocky. And every year, as the soil freezes and thaws, more rocks are pushed to the surface. This leads farmers to say that their fields “grow rocks.”

9. SCARS ON ROCKS 

Skreeeeech! It’s worse than nails on a chalkboard. A hulking, weighty glacier drags along loose pieces of rock, grinding them against the bedrock and carving long scratches. These markings, called glacial striations, show us the direction of a glacier’s flow. In some places, you can even track successive glacial movements through overlapping scratches.

10. WORM-FREE WOODS 

In many places where glaciers roamed, the native worms disappeared completely. All of that scouring wiped away plants, soil, and even earthworms, leaving the land pretty barren. Once the ice melted, the forests that sprung back up in the rubble were earthworm-free.

In colonial times, however, people shipped over plants—and earthworms—from Europe. Those imported wrigglers have infiltrated forest soils. They devour the top layer of material, making it hard for certain types of plants to grow. It’s not clear how forests will change long-term as those worms continue to munch. 

11. LONG ISLAND 

Imagine a bulldozer pushing up some dirt, then backing away, leaving that pile of soil behind. Glaciers do essentially the same thing. They create piles of debris called terminal moraines. Long Island is one such feature, and it marks the end point of a glacial advance. Compared to Manhattan, Long Island doesn’t have many big, tall buildings, and that’s partly because it’s built on unstable rubble. Cape Cod, Martha’s Vineyard, and other famous coastal features are also moraines.

12. CHANNELED SCABLANDS

This landscape in Washington State doesn’t have the most appealing name. It’s a scarred place with little soil. That’s because it was scoured clean by a cataclysmic flood.

Glaciers often blocked up rivers, creating dams of ice that led to the formation of enormous lakes. But as the planet warmed, those ice dams broke—often catastrophically. One ice-dammed lake was glacial Lake Missoula. During the dam's most catastrophic failure 20,000 years ago, water rushed out at 10 times the combined flow of all the rivers in the world. When the water rushed across dry land, the result was the Channeled Scablands. 

13. ANIMALS IN WEIRD PLACES 

Ice sheets change the landscape so much that animals wind up in strange places. For example, fish that normally spend part of their lives in the ocean can become permanently trapped in lakes, as is the case of the Killarney shad in Kerry Lake, Ireland.

And in Canada’s St. Lawrence River, there’s a remnant population of an Arctic whale—the beluga. During the last glaciation, ice sheets blanketed much of the beluga’s northern range, pushing the species southward. As things warmed up, a few whales decided to stay.

14. PLANTS IN WEIRD PLACES

There’s something strange about the plants along parts of Lake Champlain, a huge body of water that sits between New York and Vermont. Wander along its shores and you might see certain plants that really belong at the seaside, such as the beach-pea. These are leftovers of a time when the ocean reached much farther inland. As the glaciers melted back, the land in that region was squished down so much that a tongue of ocean flooded in, creating a nice home for seashore plants. 

15. BUNKER HILL

The Battle of Bunker Hill was the American Revolution’s first major battle—and it took place on a glacial hill.

In 1775, American colonial militiamen and British soldiers battled near Boston. Though the British won the fight, they were stunned by the colonists’ ferocity. It was an important fight that helped set the tone for the rest of the Revolutionary War.

The battle is named for Bunker Hill, but here’s a bit of trivia: Most of the fighting took place at nearby Breed’s Hill. Both of these landforms are glacial features called drumlins. They’re teardrop-shaped hills, and we’re still not exactly sure how they form. One thing is clear: The glaciers of the distant past shaped some of the most important events in human history.

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science
Geological Map Shows the Massive Reservoir Bubbling Beneath Old Faithful
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Yellowstone National Park is home to rivers, waterfalls, and hot springs, but Old Faithful is easily its most iconic landmark. Every 45 to 125 minutes, visitors gather around the geyser to watch it shoot streams of water reaching up to 100 feet in the air. The punctual show is one of nature’s greatest spectacles, but new research from scientists at the University of Utah suggests that what’s going on at the geyser’s surface is just the tip of the iceberg.

The study, published in the journal Geophysical Research Letters, features a map of the geological plumbing system beneath Old Faithful. Geologists have long known that the eruptions are caused by water heated by volcanic rocks beneath the ground reaching the boiling point and bubbling upwards through cracks in the earth. But the place where this water simmers between appearances has remained mysterious to scientists until now.

Using 133 seismometers scattered around Old Faithful and the surrounding area, the researchers were able to record the tiny tremors caused by pressure build-up in the hydrothermal reservoir. Two weeks of gathering data helped them determine just how large the well is. The team found that the web of cracks and fissures beneath Old Faithful is roughly 650 feet in diameter and capable of holding more than 79 million gallons of water. When the geyser erupts, it releases just 8000 gallons. You can get an idea of how the reservoir fits into the surrounding geology from the diagram below.

Geological map of geyser.
Sin-Mei Wu, University of Utah

After making the surprising discovery, the study authors plan to return to the area when park roads close for the winter to conduct further research. Next time, they hope to get even more detailed images of the volatile geology beneath this popular part of Yellowstone.

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Big Questions
Just How Hot Is Lava?
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Like the bubbling cheese of a pizza consumed too quickly, lava has been anointed as one of the most scorching substances on Earth. But just how hot is lava? How quickly could it consume your flesh and destroy everything in its path?

You may already know that lava is actually molten rock that oozes or spurts out of volcanoes because of the extreme temperatures found miles deep in the Earth. As the rocks melt, they begin to rise toward the surface. (Lava is typically referred to as magma until it reaches the surface.) As you can imagine, the heat that's needed to melt rock is pretty staggering. Cooler lava—relatively speaking—could be around 570°F, about the same as the inside of your typical pizza oven. On the extreme side, volcanoes can produce lava in excess of 2120°F, according to the United States Geological Survey.

Why is there so much variation? Different environments produce different chemical compositions and minerals that can affect temperature. Lava found in Hawaii from basalt rock, for example, tends to be on the hotter side, while minerals like the ones found near the Pacific Northwest's Mt. Saint Helens could be a few hundred degrees cooler.

After lava has erupted and its temperature begins to lower, it will eventually return to solid rock. Hotter lava flows more quickly—perhaps several feet per minute—and then slows as it cools, sometimes traveling only a couple of feet in a day.

Because moving lava takes its sweet time getting anywhere, there's not much danger. But what if you did, in some tremendously unfortunate circumstance, get exposed to lava—say, by being thrown into a lava pit like a villain in a fantasy film? First, you're unlikely to sink rapidly into it. Lava is three times as dense as water and won't simply move out of the way as quickly. You would, however, burn like a S'more at those temperatures, even if you wouldn't quite melt. It's more likely the radiant heat would singe you before you even made contact with the hypothetical lava lake, or that you'd burst into flames on contact.

Because lava is so super-heated, you might also wonder how researchers are even able to measure its temperature and answer the burning question—how hot is lava, exactly—without destroying their instrumentation. Using a meat thermometer isn't the right move, since the mercury inside would boil while the glass would shatter. Instead, volcanologists use thermocouples, or two wires joined to the same electrical source. A user can measure the resistance of the electricity at the tip and convert it to a readable temperature. Thermocouples are made from ceramic and stainless steel, and both have melting points higher than even the hottest lava. We still don't recommend using them on pizza.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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