Tectonic plates
For about 35 million years, tectonic plates have been converging along the present-day Oregon Coast causing the seafloor to sink. The ocean-floor is a small, dense plate named the Juan de Fuca Plate. The Juan de Fuca Plate is being pushed under the more buoyant continental North American plate, causing the continent to buckle and rise, forming the Coast Range, between the City of Portland and the Pacific Ocean.

As the Juan de Fuca Plate plunges under the continental plate and towards the earth’s center, the plate's crust heats up and becomes liquid rock, called magma. Some of the magma gets returned to the earth’s inner layers, while some rises and forms volcanic mountain ranges. This is what creates the Cascade Range, and forms area volcanoes like Mount Hood. The Cascades are much taller than the Coast Range, so the precipitation they receive is often in the form of snow. The elevation of this mountain range also facilitates the storage of water in the form of glaciers.

The large Pacific Ocean Plate has been moving North and dragging the western edge of North America along. This plate has been moving 2 inches per year for the past 10-15 million years, which is evidenced by the San Andreas Fault in California, as well as the northern jog the Columbia River makes just west of Portland. 

Volcanic Eruptions
Long before volcanoes formed the Cascade Range, a series of volcanic eruptions from the Grande Ronde Volcano occurred in northeastern Oregon (12-17 million years ago). Copious amounts of basalt flowed west across northern Oregon, laying the foundation for the series of scenic stacks and arches now seen along the coast of Oregon (e.g. Haystack Rock). These layers of basalt, known as Columbia River Basalt, form the bedrock for most of the region and can be easily seen in the Columbia River Gorge. 

Massive Floods
A series of catastrophic floods deposited large amounts of gravel and sand on top of the Columbia River Basalt. The first flood deposited layers in the Willamette area were unconsolidated deposits of gravel and sand known as the Troutdale Formation. 

The second major flood event, called the Bretz (or Missoula) Floods, occurred about 15,000 years ago at the end of the last Ice Age. As the ice sheets that covered most of North America were retreating towards the poles, a large glacier from British Columbia dammed the Clark Fork River in the Idaho panhandle. This glacier formed Lake Missoula, one of the largest freshwater lakes in Earth’s history. When the ice dam melted, most of the 500 cubic miles of water from Lake Missoula rushed west and scoured the Columbia Gorge within a 2-day period. The floodwaters carried gravel, sand, silt and whatever else the raging river picked up in its path. 

The Bretz Floods also melted ice caps in its path, which caused 39 additional floods during the remainder of the Ice Age. These floods left rich alluvial (water deposited) soils in the Willamette Valley and other lowlands after the waters receded.