The answer to the above question is pretty simple - we can thank Mt. Rainier. Between 5,600 and 5,700 years ago the White/Green/Duwamish watershed was a very different place. In fact, the Duwamish didn't exist as a river and Puget Sound extended east as far as Kent and Auburn of today. The Duwamish was more of an embayment and given the depth of water here, terms like fjord were sometimes used. But then we had the Osceola mud flow, also known as a lahar.
A huge mudflow or landslide took nearly 1 cubic mile from the summit of Mt. Rainier. This one single event created 200 square
miles of land in a matter of hours, with waves of mud 20 feet to 600
feet high. This wall of mud had the consistency of wet concrete and
traveled up to 60 mph. This mudflow destroyed everything in its path,
uprooting entire old-growth forests. It hit Puget Sound with such force
and with so much material that it flowed underwater for 15 miles, maybe
farther. An area of hundreds of square miles was covered with mud and
debris up to 350 feet deep.
This was not an eruption but a mudflow that removed the top 2,000 feet from the mountain and created the modern summit that we see today. Why a mudflow or lahar? According to USGS:
"Larhars are common at Mount Rainier, because its mantle of snow and ice provides water when melted, and parts of the upper flanks of the volcano contain abundant loose, weak, hydrothermally altered rock. Hydrothermal alteration occurs where hot, sulfur-rich volcanic gases encounter groundwater. The sulfur gases dissolves into the groundwater creating sulfuric acid that attacks and leaches chemical components from the rock. Commonly, the reactions replace much of the rock with clay minerals that are weak and water saturated. The presence of abundant soft, wet clay aids in mobilizing the collapsed material, allowing it to flow like a liquid."
In my simple terms, acids and gases turn rock into clay and create a viscous substance that can literally fall off the mountain under pressure from snow and gravity. The Electron mudflow from 500 years ago was just a smaller version of the Osceola lahar. As noted in the photo that follows, signs of Osceola remain today along the White River near Greenwater.
So back to the Enumclaw Plateau. The result of the Osceola mudflow was to add from 30 to 100 feet of clay creating the plateau we now see. As we know, dense clay doesn't absorb water well and as a result, we can often see standing water across the plateau making farming difficult. More recently formed topsoil is often 1 foot or less deep. Perhaps this accounts for why today we seem to have less crop oriented farming here than we do ranching. In contrast, sediment flowing downstream from the White and Green rivers over 5,000 years have deposited lots of rich new soil into the Puyallup, Auburn and Kent valleys, making this area super rich for farming (and warehouses, unfortunately).