Exploring The Earth: Hudson's Bay And The Last Ice Age

(EXPLORING THE EARTH will be a regular series of columns that will examine various aspects of Earth Science through the lens of The Layered Earth software.  This first feature in this series examines the affect of the last ice age on Hudson Bay and how The Layered Earth software can be used to illustrate several unique features of this geological event.)

Hudson Bay, in north-eastern Canada, is one of the most prominent physical features of North America.  Today it is the second largest bay in the world, after the Bay of Bengal, east of India. 

The physical location of Hudson Bay in North America.

Sixteen thousand years ago, however, Hudson Bay lay crushed beneath an ice sheet that was 3 km (1.9 miles) thick. 

The extent of the ice sheet covering North America 16,000 years ago.

Layered Earth Software Users: To view the ice sheet covering much of North America 16,000 years ago:

  • Click on the Layers button in the bottom left corner of the screen.
  • Navigate through the following subdirectories:
  • CLIMATE → ICE COVER (insert a checkmark inside this box by clicking with your mouse inside the box)→ MAPS
  • Click the radio button for 16,000 YEARS AGO.
  • (Note: Clicking other radio buttons in this section will allow you to explore how the extent of the ice sheet has varied over time. Go ahead and explore!)
  • Click the Reset button in the tool bar (upper part of screen) in order to return to the default Earth view when you have finished exploring in this section.

This ice sheet was so heavy that it actually depressed the Earth’s crust underneath it.  Hudson Bay only finally became free of ice sometime between 8,000 – 9,000 years ago.  The removal of this immense ice burden allowed the depressed crust under Hudson Bay to slowly rebound.  This rebounding continues to this day.  Evidence of a depressed crust under Hudson Bay can still be seen today in gravity anomaly maps of the area.  The entire Hudson Bay area still exhibits a negative gravity anomaly; this means that the measured value of g, the acceleration due to gravity, is less than it should be as a result of the depressed crust.

The negative gravity anomaly in the Hudson Bay area.

Layered Earth Software Users: To view the gravity anomaly data set:

  • Click on the Layers button in the bottom left corner of the screen.
  • Navigate through the following subdirectories:
  • GEOPHYSICS → GRAVITATIONAL ANOMALY (insert a checkmark inside this box)→ MAPS

The gravity anomaly data will now appear on the globe, however, a clear view of the data is obstructed by the underlying satellite view of North America. To enhance your view of the gravity anomaly data:

  • Go to the SATELLITE IMAGERY subdirectory. Click on the checkmark in the box beside the subdirectory name. This removes the checkmark in this subdirectory and removes the satellite imagery data as active data.
  • Go to TOPOGRAPHY AND BATHYMETRY → COASTLINE AND BORDERS (insert a checkmark inside this box)
  • Click the radio button for N. Amer. States and Provinces. You should now have a clear view of the gravity anomaly data along with state/provincial outlines in North America in order to give a clearer geographical context.
  • (Small points representing stars are still visible in the background at this point. If you want to remove these then remove the checkmark in the box beside the ATMOSPHERE AND STARS subdirectory.)
  • Click the Reset button in the tool bar (upper part of screen) in order to return to the default Earth view when you have finished exploring in this section.

Clear physical evidence of the rebounding of the Earth’s crust (known as isostatic rebound) can be found by carefully examining the shore of Hudson Bay.  Once such area can be located at 56.390N, 87.950W.  You can rotate the globe manually until you locate these coordinates, or you could use the Search function

This unusual view of the Hudson Bay coastline can be found at 56.390N, 87.950W.

You should now be able to see an unusual series on lines on the land surface that are parallel to the coast.  What you are looking at is a series of old beach levels that mark the location of previous shorelines on Hudson Bay.  They are now inland because of the isostatic rebound of the crust under Hudson Bay.

Explore up and down the coast of Hudson Bay to discover other areas that nicely illustrate these previous shoreline levels.  Another particularly nice example of old shorelines can be found at 58.430N, 93.100W. 

Reference Files

Ready access to all of the views and data sets that have been mentioned in this article are available if you own The Layered Earth Software.  Each file (after it has been downloaded) can be viewed by clicking File/Open in the upper menu bar, and then following the path to its location.