Prince William Sound with its 3,000 miles of shoreline is surrounded by the Chugach Mountains to the east, west and north. Fifty-mile long Montague Island and several smaller islands form natural breakwaters between the Sound and the Gulf of Alaska. Between the barrier islands stretch underwater sills separating the Sound's deep waters from the much shallower waters of the Gulf. Deep water renewal occurs during the winter when cold winds from interior Alaska cool the surface waters causing them to sink, while the warmer bottom water rises to the surface bringing rich nutrients which support huge plankton blooms in the spring.

Millions of years of glaciation gradually carved away a coastal plateau creating the sound with its many tributary fiords and passageways, islands and rocky shores. Fewer than 10,000 people live in the three towns-Whittier, Valdez, and Cordova- and two native villages-Chenega and Tatitlek situated on the shores of the Sound. Because the Sound was formed by millions of years of glaciation, its shorelines are heavily indented by deep fiords and many smaller bays. No roads connect these communities.

The geological story of Prince William Sound and Columbia Bay begins off the coast of northern California some sixty million years ago when rain and floods, freezing and thawing eroded the continental landscape. Rivers carried loads of sediments seaward depositing them on the edges of a deep, submarine canyon. Periodic earthquakes dislodged the mud and boulders, sending massive turbidity currents-a slurryof mixed water and sediments-cascading down the canyon walls to spread out in a fan below. Partial subduction of these unconsolidated sediments slowly transformed them into sedimentary rocks. Turbidites from this period occur along the western shore of Columbia Bay.

About 50 million years ago, off the northwest coast of the American continent, the Kula and Farallon ocean plates were spreading apart, magma rose and cooled underwater forming pillow basalts. The pillow basalts and sheeted dikes composing Glacier and Growler Islands were formed in this spreading center. It is here that prospectors would seek copper, gold and silver.

Meanwhile, the Pacific plate pushed the Kula and Farallon plates northward where they encountered the North American plate. The lighter continental plate rode up over the heavier oceanic plates forcing them down into a deep subduction trench. Movement of plates along this subduction trench creates many tiny and a rare few, catastrophic earthquakes. In 1964, the plates about 12 miles beneath Miners Lake in Unakwik Inlet unlocked causing the largest Earthquake recorded in North America. The quake was so strong that it was the first earthquake since the invention of the Richter scale NOT to be recorded. It went off the scale within the first few seconds. Geologists have since calculated that it would have registered 9.2 on the Richter Scale. The earthquake was so powerful it uplifted many areas of Prince William Sound from just a few to 36 feet, while other places sank 8 feet. Growler Island and the Columbia Bay area uplifted about 4 feet. Evidence of this uplift can still be seen along the shorelines.

About 40 million years ago, the heat of the Earth's interior melted the subducted sediments forming pools of magma which periodically rose towards the Earth's surface. These plutons (named for Pluto, Greek Lord of the Underworld) cooled slowly forming granites. Granites at Granite Point and Granite Bay on the west side of Columbia Bay represent this era.

Thrust faulting from subduction in the Aleutian trench over the past 20 million years gradually uplifted the Chugach Range in the Prince William Sound area. This range at first was most probably an uplifted plateau. But the combined physical forces of uplift and glaciation created the highly sculpted Chugach Mountains. The only higher range of coastal mountains is the Andes. Links to other pages on geology: