The location of Seattle is unique insofar as it is one of the largest cities in America to be situated upon a sound. It’s not the largest city in America to have this distinction – that’d be New York City, who finds itself on the west end of the Long Island Sound.
What defines a sound is dependent upon which geological definition you use; either the Scandinavian definition, which means a strait, or the narrowest part of a straight, or the 18th century English definition, which means a sea inlet that contains islands. This is the definition used for the Puget Sound.
This definition helps explain the fact that Seattle is a coastal city located upon the Pacific Ocean, even if it is around one hundred miles or so from open sea. This is a fact that is easy to ignore at times due to looking across the sound and seeing various aspects of the Olympic Peninsula.
Puget Sound is defined as all waters of the Hood Canal, the Admiralty Inlet, the Possession Sound, and the Saratoga Passage, among others. Essentially, if one were to draw a line from Mount Vernon, to Port Townsend, Puget Sound would include all waters of the inlet from that line, down to Budd Inlet in the south by Olympia. That’s a body of water that stretches approximately one hundred miles.
Seattle is located on the Eastern Edge of the Puget Sound, and this aspect of the city is the primary, but certainly not only, defining geologic feature of the region.
All of the Puget Sound region was shaped in larger part due to the Fraser Glaciation. Evidence of glaciation is easily seen today all along the Puget Sound area. The Fraser glaciation lasted about 10,000 years and consisted of 3 periods of ice expansion (called “stades”) and 2 ice recessions (called “interstades”). The Puget Sound was shaped by the Puget Lobe portion of the Cordilleran Ice Sheet that was part of the Fraser Glaciation that shaped much of the Montana, Idaho, Washington, British Columbia, the Yukon Territory, and Alaska. The Puget Lobe portion of this ice sheet made it as far south as modern-day Olympia. As it retreated over the course of many thousands of years, it geologically shaped many of the water ways of the region, including Puget Sound, as well as Lake Union, Lake Washington, and Lake Sammamish to the east.
The retreat also helps explain the hilly nature of the region, though not so much the mountainous portions, whose shapes were created under different forces.