sentences:  Rocks of the hills include Tertiary volcanic bedrock, sedimentary bedrock, and Plio-Pleistocene sedimentary basin fill shaped by elongate domical folds.The Waldo Hills form part of the divider between the upper and lower Willamette Valley.Additionally, the Waldo Hills as part of a larger fault system of low-lying hills in the mid-valley, are the largest geological structure in the mid-Willamette Valley.