 During the third, or Vashon Glaciation, a lobe of the Cordilleran Ice Sheet, called the Puget Lobe, spread south about 15,000 years ago, covering the Puget Sound region with an ice sheet about thick near Seattle, and nearly at the present Canada-U.S. border.Since each new advance and retreat of ice erodes away much of the evidence of previous ice ages, the most recent Vashon phase has left the clearest imprint on the land.At its maximum extent the Vashon ice sheet extended south of Olympia to near Tenino, and covered the lowlands between the Olympic and Cascade mountain ranges.