During the last ice advance and retreat (in the latest Pleistocene), an extension of the Cordilleran ice sheet called the Puget lobe covered the Puget Sound region (see location map below).
The colored area of the map (at right) represents the approximate maximum extent of the Puget Lobe during this time period. During glacial maximum, the location of modern-day Seattle (near the center of this map) was beneath 3,000 feet of ice.
All of the land and waterways in this region were shaped, at least in part, by the glacial ice of the Puget lobe. Many large-scale glacial landforms are preserved in the landscape today. In this lidar*-derived map, landforms such as drumlins, kettles, eskers, and glacial stream channels can be seen. Examples of these and other landscape features are enlarged at bottom right.
Most of Washington’s population lives in this region—the glacial geology influences many aspects of daily life, including transportation, water supply systems, agriculture, and building regulations.
During the advance and retreat of the Puget lobe, drainages around the ice sheet were blocked, forming multiple proglacial lakes. The darker colors on this map indicate lower elevations and show many of these valleys. The Stillaguamish, Snohomish, Snoqualmie, and Puyallup River valleys all once contained proglacial lakes. There are many remnants of these lakes left today, such as Lake Washington and Lake Sammamish, east of Seattle.
As the Puget lobe retreated, lake outflows, glacial meltwater, and glacial outburst flooding all contributed to dozens of channels that flowed southwest to the Chehalis River at the southwest corner of this map. Remnants of these channels can be seen along the eastern and southern edge of the colored area in the map.
Present-day Lake Kapowsin and Ohop Lake both occupy one of these channels. Today, the Chehalis River flows through a wide valley that was largely sculpted by ice-age meltwater.