Reflections 2010
Series 25
December 15
The Ice Sheet from the Great Lakes to the Hudson

 

I find that geography and history are the cornerstones of travel. Geographically, I’ve been reaching out to new places, and continue to do so, but what about history and historical time periods? I’ve finally clarified in my own mind which time periods I enjoy most. It depends if the reference is earth history or human history. As for earth history (geology/earth science), the older the better. I loved talking about how the Mediterranean got filled with water and how Niagara Falls was formed (2005/14).

 
 

As for human history, it’s just the reverse in my mind, the newer the better. Ancient history (Mesopotamia) is of mild interest to me, as is Rome and Greece. The Middle Ages (5C to 14C) also interest me mildly, although I get really fired up when travel routes are involved, such as Marco Polo’s (1271 on). But I’ve finally self-analyzed my interests to come to the realization that it’s the Modern Age that I’m really passionate about, the contemporary period after the Middle Ages. It’s hard to distinguish between explorers and travelers. And again here, as with Marco Polo, the more travel-oriented the history is, the better. I start enthusing about the explorers crossing the oceans (and beyond) in the 15C (Columbus, Cabot, da Gama), 16C (Magellan, Verrazano, Drake), 17C (Hudson, Champlain, Tasman), 18C (Bering, Cook, von Humboldt), 19C (Lewis & Clark, Livingstone & Stanley), 20C (Amundsen, Byrd, Earhart). This is an incomplete list, but quite a few of these names have appeared in these pages, including most recently Hudson & Champlain, Lewis & Clark, Livingstone & Stanley.

 
 

Yet, to cut a time-line. I would start with the year 1600 and run through to the present, roughly the last four centuries. The beginning of this period covers the Dutch explorations, and we’ve already talked quite a bit about the Dutch settling New Amsterdam starting right after that date, and about Dirk Hertog and Abel Tasman in the discussions of Australia.

 
 

This next project is twofold and covers both time periods above. My original intent in the first case was to research and discuss the confusing channels and islands in New York Harbor and make sense of them. I find the results fascinating, but the story is incomplete without first discussing the last Ice Age and its ice sheet, nor is it complete without discussing other areas in northeastern North America affected by it, such as (1) the Great Lakes and the Hudson and (2) the Outer Lands (Staten Island to Cape Cod). These topics will take several postings, culminating in a discussion of New York Harbor. Then, as a follow-up afterward, we’ll discuss how the sectors of the New York metropolitan area fit together--or don’t.

 
 

The Ice Sheet   Whenever this subject comes to mind, I visualize it in terms of my own experience once with ice, snow, meltwater, and general chaos. I remember it quite clearly, even though I was just eight years old. The New York City area was hit in 1947 by its worst blizzard since the infamous Blizzard of 1888, and one that wasn’t to be surpassed in snowfall depth until 2006. I just checked online to see that it happened during the two days after Christmas, 26-27 December 1947, when 26.4 in (671 mm) of snow fell, and then was blown into huge drifts. We lived on an urbanized block of Jerome Street in the East New York neighborhood of Brooklyn, and I remember walking down a path along the shoveled sidewalks with the snow up to my shoulders at least.

 
 

But the memory and image goes beyond that. For two days at least, no traffic moved on the streets, and as the sun came out and temperatures rose, the kids emerged from indoors ready for fun in the vast amount of melting snow and flowing meltwater. We played beyond the sidewalk and curb, well out into the middle of Jerome street. We built, moved, and destroyed snow mountains, formed meltwater lakes and then burst the dams to send handcrafted rivers down the street. It was a period of constructional creativity amid large-scale general chaos.

 
 

My memory is to me the microcosm (small world) version of the huge macrocosm (large world) of the last ice sheet over North America as it moved mountains, altered the course of rivers, formed and destroyed islands and lakes, then flooded those freshwater lakes with seawater as the sea level rose due to the diminishing ice sheet, making the lakes part of the ocean. It’s a heady image of chaos resulting in a massive alteration of the landscape.

 
 

Today we primarily talk of glaciers, such as in our discussion of the Swiss Alps (2008/15). But picture glaciers in adjacent valleys that grow so large as to surpass in height the mountains between them, and then connect into a pattern of multiple glaciers over the landscape and form an ice sheet.

 
 

There have been four major periods of glaciation, called Ice Ages, each the cause of major extinctions. Geologists agree that one of these Ice Ages, over 650 million years ago, was so complete that the entire Earth, or at least most of it, froze over, giving rise to the reference “Snowball Earth”. Each Ice Age had multiple periods of advancing ice (“glacial periods”) and receding ice (“interglacial periods”). The Quaternary (Fourth) Ice Age began 2.58 million years ago and we are still in it (ice sheets still exists in Antarctica, Greenland, and Canada, for instance), but we are in an interglacial period. The last glacial period began roughly 70 millennia (70,000 years) ago, reached its maximum extent 20 millennia ago, and ended 15 millennia ago. This Laurentian (or Laurentide) Ice Sheet is named after where it apparently was formed, in the Laurentian Mountains (French: les Laurentides) in Québec north of the Saint Lawrence River. This is the ice sheet we’ll be discussing.

 
 

The Laurentide Ice Sheet is shown here in its Last Glacial Maximum (furthest extent of glaciation) about 20 millennia ago. In Eurasia it covered the British Isles, Scandinavia, Central Europe, and much of Siberia. In North America it covered Greenland, all of Canada, only part of Alaska, and the northern United States, the area in which we’ll be concentrating. Notice in particular how it covers the eastern US from the Great Lakes to New York City on the east coast.

 
 

The ice sheet was formidable, up to 3-4 km (1.9-2.5 mi) thick inland, but about 400-500 m (1300-1600 ft) thick along its southern edge. The growing ice, although formed of fresh water, got its water from the sea through evaporation and precipitation, and the ice sheet caused a global sea level drop of about 120 m (390 ft) (add ice, lose water; lose ice, add water). It’s not gone. A remnant of the Laurentide Ice Sheet remains in Canada and is Canada’s oldest ice (20 millennia old). In Canada’s Arctic it covers central Baffin Island, Canada’s largest island and the fifth largest in the world. It is currently known as the Barnes Ice Cap.

 
 

Prior to the Ice Ages, most northern rivers in North America flowed north into Hudson Bay. The ice sheet blocked and diverted them. The most recent Ice Age added the “finishing touches” to the modern landscape. The glaciers sculpted the landscape by eroding and transporting large quantities of rock and sediment, by blocking and altering the course of rivers, and forming terminal moraines. A moraine is an accumulation of glacial debris, such as rocks and boulders. The longer a glacier stays in one place, the more the debris can accumulate. A terminal moraine is at the end of a glacier (or ice sheet) and shows its farthest advance; a recessional moraine is left when the glacier pauses during its retreat (see Long Island below). Major effects of the ice age were (1) erosion and deposition of material (see Outer Lands below); (2) modification of river systems (see Hudson below); (3) creation of lakes (see proglacial lakes such as Lake Connecticut below); (4) changes of sea level, including coastal flooding (see New York Harbor and Outer Lands below). In regard to proglacial lakes, note that an isostatic depression is the sinking of part of the earth’s crust due to the weight of a glacier, and if meltwater collects and is trapped in such a depression this can also form what is known as a proglacial lake. Such a lake can also be formed during the retreat of a melting glacier by the damming action of a moraine or ice dam. As the ice sheet receded 10 millennia ago, large proglacial lakes were a widespread phenomenon in the northern hemisphere, such as Lake Albany in the Hudson valley, Lake Connecticut in Long Island Sound, and Lake Cape Cod in Cape Cod Bay.

 
 

Take note of this landscape produced by a receding glacier, (Image by Hans Hillewaert) especially (1) the meltwater coming from the glacier forming a river. Consider the southbound rivers in the US, including where the Missouri River-Ohio River-Mississippi River exited the ice sheet, and all the southbound rivers in the northeast US; (2) the terminal moraine such as on Long Island, with the fine material in the outwash plain south of it (the sandy South Shore beaches) and the rough gravel having been beneath the ice sheet (the rocky North Shore beaches); and (3) the proglacial lakes forming from meltwater at the foot of the glacier (Lake Connecticut that became Long Island Sound). These chaotically changing lakes and rivers are my Blizzard of 1947 on a spectacularly grand scale.

 
 

The Great Lakes   The geology that created the upper Great Lakes took place 1.1 to 1.2 billion years ago when tectonic plates split and formed a valley that became Lake Superior. Later on, 570 million years ago, a fault line appeared and created the basis for Lakes Erie and Ontario, and the Saint Lawrence River. The lakes as we know them were then formed about 10 millennia ago after the Laurentide finally receded, leaving behind a huge amount of meltwater forming the lakes. The Great Lakes are essentially freshwater seas. They are the world’s largest group of freshwater lakes by total surface and volume and hold 21% of the earth’s surface fresh water. When talking about the Lakes in reference to Niagara Falls (2005/14) we first pointed out that, although people usually consider there are five lakes, there are actually only four, since what popularly is referred to as “Lake Michigan” and is really just a lobe of what is popularly referred to as “Lake Huron”. A scientifically accurate name for the combined lake (including the huge Georgian Bay to the east) is Lake Michigan-Huron.

 
 

[It is possible to keep these two realities, factual and popular, in one’s mind simultaneously, since we do it all the time. When we watch the sun on the western horizon, the factual reality is that we (and the Earth) are moving backward away from it, so we see less and less of it, but the practical reality seems instead that it’s the sun that’s moving. This practical reality still causes us to talk about a “sunset” and about “the sun going down”. Similarly, we can appreciate the dual reality of both the popular conception of five Great Lakes, as shown in the map above, and also the scientific fact that Lake Michigan-Huron is one single lake, making a total of four.]

 
 

In size, if you consider there are five, each one is smaller as you progress. Using Superior as a size benchmark of 100%, it would not only be the highest in altitude, as the name implies, but also the largest. Huron is 73% as large, Michigan, almost the same size as Huron, 71%; Erie 31%; Ontario 24%.

 
 

But as mentioned, Lake Michigan-Huron, including huge Georgian Bay, is in reality a single multi-lobed lake, since all its water is on the same level, and there is no river connecting them as with the others, a prerequisite for all chains of lakes. Rather the lobes are connected by the 8 km [5 mi] Straits of Mackinac [MA.ki.naw], through which waters flow either east or west. Using that scenario, then Michigan-Huron is the largest, being 143% the size of Superior, which is second in size, followed again by Erie and Ontario.

 
 

The lakes’ names are derived from Native American words, except for Lake Superior, which is simply a poor English translation of the French lac supérieur, more properly “Upper Lake”, referring to its altitude at the beginning of the chain. The original Ojibway name for Lake Superior is Gichigami, which Longfellow used in the form Gitche Gumee in his 1855 “Song of Hiawatha”:

 
 
  By the shores of Gitche Gumee,
By the shining Big-Sea-Water . . .
 
 

This height of the lakes is very significant and is best illustrated by this Great Lakes System Profile. Measurements are only given in feet, but the numbers can be used for comparisons. For instance, notice how deep some of the lakes are. Subtracting Superior’s two figures shows that more than half of its depth is below sea level. This chain of four major lakes is connected by three river systems. The first is the Saint Mary’s River, which is steep and which has the famous Soo Locks for maritime passage. The second is the combined Saint Clair/Detroit Rivers (passing through tiny Lake Saint Clair). No locks are needed here as the total drop in height from Michigan-Huron is minimal as water enters very shallow Lake Erie. Finally, the third river connecting the lakes is the Niagara River and . . . WOW! What happened to the upper part of Lake Ontario? Well, in a manner of speaking, it floated down the Hudson River (below).

 
 

All the lakes had been preceded by proglacial lakes of varying sizes and shapes, but the one of interest to us is Glacial Lake Iroquois, a prehistoric version of Lake Ontario that existed about 13 millennia ago. It was larger and deeper than Lake Ontario because its surface was about 30 m (100 ft) higher. Remnants of its former shoreline can be seen surrounding Lake Ontario in Toronto and along Ridge Road in New York State. The Niagara River flowed into Lake Iroquois at close to an even level, just like the other short rivers that connect the Great Lakes. Since the fault line that formed Lakes Erie and Iroquois/Ontario also formed the Saint Lawrence River, this river would have been the natural outlet for the Great Lakes to the ocean via the huge Gulf of Saint Lawrence. However, the Laurentian Ice Sheet blocked the northeastern end of Iroquois/Ontario, preventing an outflow down the Saint Lawrence, but also artificially maintaining the height of Lake Iroquois. With this height, the Niagara River flowed calmly into it from Lake Erie. Here is a map of how Lake Iroquois might have looked, superimposed over its remnant, the current Lake Ontario.

 
 

The ice sheet is shown in stylized form blocking a potential Saint Lawrence outlet. The Niagara River flows in at an even level. Lake Iroquois being larger, apparently one of the Finger Lakes, Lake Cayuga (and perhaps more) was a slender arm of it. But most importantly, a much larger arm is located to the southeast where the current Oneida Lake is, with a channel down the valley of the Mohawk River, leading to the Hudson.

 
 

Finally, in the later stages of deglaciation 13,350 years ago, there was a catastrophic draining of Lake Iroquois, reducing it in size to what is today Lake Ontario, and this outflow was through what would become the modern Mohawk Valley. This enormous discharge of water on the way to the Hudson caused local deep scour features. Given this drop in lake level from Iroquois to Ontario, the Niagara River no longer flowed smoothly into Iroquois but dropped precipitously into Ontario in the form of a proto-Niagara Falls, at the end of the escarpment (the Iroquois lakeshore). These falls eventually cut back the river bed to the south halfway to Lake Erie to the point where Niagara Falls is today. At a later date, outflow from Lake Ontario did make its way down the Saint Lawrence, where it remains today, cutting off permanently the outflow down the Mohawk and Hudson.

 
 

The Hudson   So it was the Hudson, not the Saint Lawrence as today, that was the original outlet for the Great Lakes. This shows the area of the Hudson River Watershed (Map by Kmusser). As you look at it today, the Hudson starts quite small up north at Mount Marcy and is joined then by the Mohawk, its largest tributary, which enters it at about Troy, just upstream from Albany, the capital of New York State. In its entirety, the Hudson River flows from north to south for 507 km (315 mi) from Mount Marcy to New York Harbor. But as you look at it with yesterday’s eyes, you see the reverse: it’s obvious that the main branch of the “Hudson” draining the Great Lakes had been what is today the Mohawk, and the section from Mount Marcy was the tributary. This is the topsy-turvy world caused by the ice sheet.

 
 

But there’s more. Glacial Lake Albany was a proglacial lake that existed between 15 and 12.6 millennia ago. It was created when an ice dam formed in the lower Hudson Valley, holding back meltwater from a retreating glacier plus water from the Mohawk River and others. The lake covered the Hudson Valley for some 260 km (160 mi) from Glens Falls in the north to Newburgh in the south. This would indicate that about 1/3 of Lake Albany was north of the Mohawk confluence with the Hudson and 2/3 south, but didn’t reach the present New York Harbor. The remainder of Lake Albany drained about 10.5 millennia ago down the Hudson River.

 
 

We’ll talk of the mouth of the Hudson and New York Harbor separately later, but one final interesting fact about the Hudson should be brought up here, a fact based on the glacial history of the Hudson, including the massive coastal flooding resulting from the melting ice sheet that has affected the coastline of this section of the US Northeast: the lower Hudson is NOT a river.

 
 

A submergent coastline is one that has been inundated by the sea due to rising sea levels. Features of a submergent coastline involve drowning, such as drowned glaciated valleys, or fjords, drowned river valleys, or rias, and drowned river mouths, or estuaries.

 
 

As we discussed in New Zealand (2009/9), fjords and rias appear to be similar, but are formed differently. Both are long, narrow inlets, drowned by the sea, but a fjord is formed when a glacier cuts a U-shaped valley by means of erosion. When the glacier recedes, the sea floods in. A ria is a natural river valley (non-glacial) along the coast which was flooded only by rising sea levels reaching inland.

 
 

Rias are to be found, among other places, along the submergent south coast of England (Portsmouth Harbour, Southampton Water); the previously described northern end of New Zealand’s South Island (Marlborough Sounds, 2009/9); Sydney Harbour (2010/19). Rias in the US include, in the west, both lobes of San Francisco Bay exiting the Golden Gate, and in the east, Chesapeake Bay, Delaware Bay, and Narragansett Bay in Rhode Island. [We discussed in 2009/21 how the Chesapeake is the flooded lower portion of the Susquehanna, into which the Potomac and other rivers used to flow directly; they now flow into the Bay. Narragansett Bay we’ll discuss shortly.]

 
 

Glaciated regions featuring fjords include (among others) Norway (2006/5); Iceland; Greenland; the Canadian Arctic Islands; the southwest coast of New Zealand’s South Island; Alaska’s Glacier Bay and Lynn Canal (2005/13); and the Hudson River (most clearly seen in the Palisades).

 
 

The Hudson Fjord was created by the ice sheet 26 to 13.3 millennia ago. The tides in its estuary influence the flow almost to the confluence with the Mohawk. Because of the tides, winter ice flows both upstream and downstream, depending on the tides. An early Indian tribe living on the river had a name for it that translates as “river that flows both ways”.

 
 

In the case of the Hudson we have a contradiction with popular conception, resulting in another dual reality, as in the total number of Great Lakes. The “every-day” reality is that the Hudson is one of the longest “freshwater” rivers in the US, flowing from Mount Marcy all the way to New York Harbor. The other, factual, reality is that only the upper part of the Hudson is a totally freshwater river, the balance being a tidal estuary set in a glacially-formed fjord. The northern part of this estuary has brackish water and the southern part salt water. Phrased differently, both the Mohawk and (foreshortened) Hudson merge, and shortly thereafter together enter the Hudson Fjord. One can live with both realities, as long as both are respected.

 
 

Consider this to fortify the factual reality of the Hudson: Sydney Harbour, a ria, is the flooded lower (eastern) end of the Parramatta River, yet the common conception is that the River is just a tributary of the Harbour, entering it from the west. In a similar fashion, the (foreshortened) Hudson River enters the Hudson Fjord from the north. This may not be the every-day conception of the Hudson, but is the scientifically accurate one. We can live with both.

 
 
 
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