The bedrock of Squak Mountain is made up of rocks that formed from sediments carried by rivers into an ancient coastal estuary, where their increasing weight caused the land to sink, only to be buried by even more sediments. In time, the sedimentary layers accumulated more than one mile of thickness. As the sediments slowly collected, sub-tropical forests flourished on the landscape. Trees died and were buried before they began to decay, slowly converting into peat. Further burial and heat, possibly from regional molten magma intrusions in the Cascade Mountains, reduced the water, methane and carbon dioxide in the peat, and changed it into coal. The coal seams are found today in a layer of sedimentary rocks that geologists have named the Renton Formation.