North American Glaciation

By Jakob Hamann

Introduction

Large parts of North America have been covered by glaciers and ice sheets in the past. Over roughly the last 2.6 million years – an interval known as the Quaternary Period¹ – glaciers advanced and retreated repeatedly as climate shifted between colder and warmer conditions². During colder intervals, ice sheets expanded across much of Canada and into the northern United States, with major impacts on climate, sea level, ecosystems, and early human populations.

Major ice sheets of North America

During the maximum extent of the most recent glacial period, known as the Last Glacial Maximum around 20,000-26,000 years ago, three large ice sheets existed across North America³. Together, these ice masses covered large parts of the continent and shaped much of the North American landscape we see today.

Laurentide Ice Sheet

The largest was the Laurentide Ice Sheet, which covered most of present-day Canada and extended south into the northern United States. At its greatest extent, the Laurentide Ice Sheet was the largest ice sheet on Earth outside Antarctica³.

Cordilleran Ice Sheet

To the west, the Cordilleran Ice Sheet developed over the mountainous regions of western Canada and Alaska. Its extent was strongly controlled by topography, with fast-flowing outlet glaciers draining through major valleys and fjords⁴.

Innuitian Ice Sheet

In the high Arctic, the Innuitian Ice Sheet covered many islands of the Canadian Arctic Archipelago. This ice sheet was smaller and more fragmented than the Laurentide Ice Sheet, with modern ice caps and mountain glaciers on the Queen Elizabeth Islands of Arctic Canada considered remnants of this former ice cover⁵.

Growth and Decay

Over the Quaternary Period, these ice sheets waxed and waned, paced by the Milankovitch Cycles, in beat with the Eurasian, Patagonian, Greenlandic and Antarctic ice sheets.

How do we know?

Scientists reconstruct past glaciation in North America (and globally) by combining several types of evidence preserved in the modern environment. Much of the most direct evidence comes from the physical record left behind by glaciers and ice sheets. This includes glacial landforms such as moraines, streamlined features formed beneath moving ice, meltwater channels, and widespread glacial sediments, which indicate where ice sheets once existed and how they flowed⁷.

To place this geological evidence in time, a range of dating techniques is used. Common methods include radiocarbon dating⁸, optically stimulated luminescence dating⁹, and cosmogenic nuclide dating¹⁰,all applied to materials associated with glacial deposits, landforms, or surfaces exposed following ice retreat. Together, these approaches provide age constraints on the timing of glacial advance and retreat.

This geological and chronological data are then combined with numerical ice-sheet and climate models. These models are used to reconstruct where ice sheets existed more broadly and how their extent changed under different past climate conditions¹¹. Such reconstructions help place modern glacier and ice-sheet change in context and inform understanding of how future ice loss driven by ongoing human-driven global warming may affect sea level, freshwater resources, agriculture, and societies worldwide.