Throughout the Quaternary period, the Adirondacks have been affected by advancing and retreating ice sheeets.  During the time of advancing ice sheets, the snow of winter did not completely melt.  Because of this, snow compunded on top of older snow year after year and the accumulating snow packed the lower layers into ice.  These layers eventually became thousands of feet thick, softening the lower layers and causing them to flow, creating a land glacier (Snow Patrol).

As dense glacial ice piles up, a glacier is formed.  The ice begins to move under its own weight and pressure.
(Paleontological Research Association)used by permission

Much of the current topography of the Adirondack region is a result of extensive glaciation during the last Ice Age.  There have been at least four periods of advancing ice with three interglacial periods:

1,800,000 to 1,650,000 years ago
the Nebraskan
advancing ice
the Aftonian
retreating ice
900,000 to 750,000 years ago
the Kansan
advancing ice
the Yarmouth
retreating ice
400,000 to 250,000
the Illinoian
advancing ice
the Sangamon
retreating ice
100,000 to 10,000 years ago
the Wisconson
advancing ice

The movement of the ice sheet over North America
(Paleontological Research Association)used by permission

The last ice sheet to cover the area, named the Labrador ice sheet, had a tremendous effect on the region.  This ice sheet wiped out any traces of the pressence of its predecessors (Cushing).

Extending from the north and northeast, the Labrador ice sheet was separated into two ice streams by the opposing Adirondack elevation.  One stream followed the St. Lawrence Valley and the other was forced to the south, traveled along the western side of the mountains, and entered the Mohawk River Valley (Cushing).  While this advancement was happeing, alpine glaciers were forming in the mountains and carved the upper slopes (Snow Patrol).  Eventually, the ice sheets increased in thickness and overtook the entire area, where it stayed for a very long time (Cushing).  This sheet extended well southward of the Adirondacks and into central Pennsylvania (Naslund).

Advancement of the ice sheet over the northeast
(Paleontological Research Association)used by permission

Eventually, the climate changed and a reversal began.  The sheet was thinnest over the higlands, so this area was uncovered first.  Again, the ice was split into two sections and the two retreated in the opposite manner in which they advanced (Cushing).  As the ice retreated, it left a different topography than what it covered.  Glacial wear caused smmoothed slopes and  rounded summits.  Valleys were left with an abundance of deposits and lakes and streams were changed by erosion and deposition (Cushing).

The rounded rocks of a stream in the Adirondacks
(National Geographic)used by permission

Various other changes occurred throughout the region:

- Kettle Holes and Kettle Ponds were formed.  As the ice retreated, big chuncks were broken off of the main glacier.  They were buried with sediments and as they melted left large depressions forming a kettle hole.  If these depressions were below the water table, they filled with water to form a kettle pond.

- As the ice sheets melted, rivers were formed that carried sand and gravel into new areas and deposited them onto new floodplains.  As directions and velocities changed, layers of different sized particles were built upon one another, creating the outcrops that can be viewed in the valleys today.

- Till was spread all around the Adirondacks as a result of direct deposit from the glaciers.  The till had not been transported by streams and therefore had not experienced the amount of erosion other particles had.  These deposots were  jagged  and rough (Snow Patrol).

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