The last great Cascadia subduction zone earthquake occurred on January 26, 1700. This earthquake caused sudden subsidence (down dropping) of areas along the coast from northern California into southern British Columbia Canada producing the ghost forests. This earthquake also caused a tsunami that inundated low-lying areas of the Pacific Northwest coast. In some areas, tsunami sand deposits overlie the forest floor of the ghost forest and in turn are overlain by intertidal muds and clays deposited since 1700. This succession of geologic layers (organic-rich forest floor – tsunami sand – intertidal mud and clay) has been referred to as the “three-layer cake” of Cascadia tsunami geology. In this guided-inquiry activity, students can examine a large photo of Cascadia tsunami geology that spectacularly records the 1700 great Cascadia earthquake and tsunami.