When the Ice Age Floods roared into the Portland Basin they deposited gravel bars and scoured around topographic highs like the ‘Boring’ volcanoes (Powell Butte, etc.) and the Portland Hills. As they flowed on toward the ocean they were obstructed downstream at Kalama Narrows, which caused the flood waters to back up in temporary Lake Allison that filled the Willamette Valley up to ~400 feet deep all the way south to Eugene.

In the quieter areas of Lake Allison the sediment-laden waters deposited large volumes of rich Palouse soils that were stripped from upstream areas of central Washington and now support vibrant agriculture throughout the Willamette Valley. But as the muddy waters rushed into and later back out of narrow passages they scoured out depressions that are now filled by peaceful lakes like Lake Oswego. 

Many icebergs floating in the flood waters became stranded on the edges of Lake Allison as the flood waters receded, where they melted and deposited exotic ‘erratic’ rocks embedded in them (even a meteorite) that were carried from the ice sheet far to the north.