The Iromohawk River  

BHBL and Vicinity - Geological History

New York State, like the rest of the Northeast, owes it characteristic landscape to the last of the powerful glacial ice sheets that crushed and carved out the land on a massive scale.

The two-mile thick North American Laurentide Ice Sheet covered hundreds of thousands of square miles throughout the majority of Canada and northern United States. The maximum extent of glaciation was approximately 22,000 years ago and created much of the surface geology of southern Canada and the northern United States, leaving behind glacially scoured valleys, kettles and kames (see geo terms).

New York State's rivers, lakes and valleys, soils sands and clays are all the leftover results of the massive weight and subsequent melting retreat of the huge ice sheet. Fair Use Notice

- The northeast extent of the Laurentide Ice Sheet during the Late Wisconsinan Stage-
Laurentide Ice zSheet newyork0icesheet

As the Laurentian Glacier retreated, it blocked the outflow of Glacial Lake Iroquois. Instead of flowing down the St Lawrence Valley, it flowed down the Mohawk Valley.

Glacial Lake Iroquois

fairchild CoverSource - Credit

A large percentage of this page was based on the the writings of Herman Le Roy Fairchild found in his book: POSTGLACIAL FEATURES OF THE UPPER HUDSON VALLEY Cite> Written in 1917, it is an interesting in-depth examination of the geology of the Capital District and prompted this very slimmed down online version.

 


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Origin and Metamorphosis of the Mohawk River

- click on a map thumbnail to see the map -

1 - The Glaciomohawk River is born

Map Thumb The huge Hudson Ice Block reaches south past Albany. The massive Laurentide Ice Sheet is slowly retreating and meltwater builds up in Lake Iroquois. Since the St. Lawrence Valley is blocked by the Ice Sheet which towers up a mile or more, the Glaciomohawk River is born as massive amounts of runoff crashes its way down the east-west valley between the Adirondacks and the Catskills.

2 - Sand Deposits Accumulate

Map Thumb As the Hudson Ice Block retreats northward, the Glaciomohawk River steadily drops ever-increasing amounts of glacial scrapings (sand, gravel, rocks, etc.) as it enters Lake Albany. Large deltas and sand deposits are formed where various streams of meltwater meet Lake Albany.

3 - Uplift Causes River Diversion

Map Thumb The land east of present day Schenectady slowly uplifts, rebounding from the massive weight of the shrinking Hudson Ice Block. Finally, the Glaciomohawk delta is filled with so much debris that—combined with the aforementioned uplift—the GlacioMohawk is diverted northward. Geologists refer to the river at this stage as the Iromohawk and the reroute it carved as the Ballston Lake - Morning Kill Channel.

4 - East Line Turn

Map Thumb Further uplift and tilting eastward turns the Iromohawk eastward at East Line. This reroute is referred to by geologists as the Ballston Lake-Round Lake Channel.

5 - Three Channels Form

At East Line, the northward coursing Iromohawk divides into three distributaries. One, northward toward Ballston Spa called the Mourning Kill Channel; the second, the Drummond Channel, leads northeast toward the site of Saratoga Lake, which was then occupied by a huge, detached ice block. The third channel, known as the Maltaville Channel, leads southeast toward Mechanicville, passing the site of Round Lake, which was also occupied by a mass of ice.

Both the Mourning Kill and Drummond Channels build up the lower sand plains at Saratoga Springs and west and southwest of Saratoga Lake. The Maltaville Channel sweeps its detritus past the Round Lake ice block and drops it in the neighborhood of Mechanicville.

East Line 3-way Diversion
East Line 3-way Diversion East Line 3-way Diversion

6 - Ice Blocks Block the Channel Flow

iceblocks thumbUneven melting of the huge retreating ice sheet leaves large scattered ice blocks. Geological studies of sand, gravel and surrounding topography reveal that huge detached masses of ice lay where both present day Saratoga Lake and Round Lake are situated.

7 - Saratoga Lake and Round Lake

Obstructing the water flow, glacial debris builds up around these large ice blocks. Once these oversized ice blocks melt, the sediment collapses, leaving a round depression, geologically referred to as a kettle hole lake.

- What is a Kettle? -
Kettle Formation Kettle
A kettle hole is formed by blocks of ice that are separated from the main glacier - perhaps the ice front stagnated or retreated or perhaps ice blocks were washed out from the glacier during a glacier flood. If conditions are right, the isolated blocks of ice then become partially or wholly buried in outwash. When the ice blocks eventually melt they leave behind holes or depressions that fill with water to become kettle hole lakes.

7 - The Ballston Lake Graben

The unusual shape and depth of Ballston Lake originated not from massive ice block stragglers (the above-mentioned kettle hole process) but from the shape of the land itself. Lying on the Saratoga-McGregor fault zone Ballston Lake is a result of a "graben". A thin slice of land was abruptly dropped in an ancient fault line creating the unusually deep but long and thin lake we know today.

What is a Graben? Saratoga-McGregor fault zone
graben Saratoga-McGregor fault

8 - Breaking Through at Rexford

Cliffs At RexfordAs the uplift of the land from the glacier rebound moved slowly westward, the northern flow of the Iromohawk was weakened. Eventually, the flow diverted straight east through the present day narrow channel at Rexford.

The high cliffs due east of Rexford are testament to the blockage that long held the course of flow northward. Even at the reduced flow of the river at this point in time, the river had enough strength to chisel it's way through.. imagine in it's early stages, just how powerful it must have been!

 
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Further Info

 
Glacial geology terms Ice Age Info Interesting Articles on Local "Graben" Lakes

Great Video

Other Reading

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