Over the past 2 million years of the Quaternany period, geologic processes have slowly shaped the landscape around Mukilteo and Puget Sound as the older Olympic Mountains eroded and broad waters, islands, coastal bluffs, and beaches formed. Six glacial episodes affected Puget Sound, with glaciers advancing south from Canada and later melting and retreating north. These glaciers carved the land, pushed soil and rock before them, and worked with water flowing beneath the ice, while meltwater flowing south later eroded much of the material left during glacial advance. The older soil deposits in Mukilteo's bluffs were emplaced about 200,000 years ago during the Pleistocene period, and the youngest about 10,000 years ago. The most recent episode, the Fraser Glaciation, lasted from about 25,000 to 10,000 years ago. During its Vashon Stage, southward-moving ice from Canada reached the Olympic Mountains, blocked the Puget Lowland, and turned it into a large lake. Water entering the lowland first filled the lake with clay and silt and then with sand as the ice approached. The bluffs south of Lighthouse Park show these differing deposits, with silt and clay laid down in quiet lake water and coarser sand and gravel emplaced by higher-energy flowing water. Large boulders in the area, called dropstones or glacial erratics, were carried into place by floating ice in the inland sea or left behind as fixed ice melted. Today's landscape preserves sediment transported and deposited during glacial advance, while open areas reflect erosion by water from melting glaciers as they retreated and wasted away. Weathering, landsliding, and erosion continue to alter the land. Puget Sound currents move sediment eroded from the uplands to form beaches, and wave action and littoral drift constantly move the beach sediments. These processes created a sand spit many years ago at the site of Lighthouse Park, and the embayment once lying between the present beach and the bluff south of the lighthouse was later filled during waterfront development to create the level ground that now makes up much of the park.