The Portland Basin is a roughly 770 square miles (1,994.3 km²) topographic and structural depression in the central Puget-Willamette Lowland. The Portland Basin is
approximately 40 miles (64.4 km) long and 20 miles (32.2 km) wide, with its long axis oriented northwest. Studies indicate that as much as 1800 feet (548.6 m) of late Miocene

Miocene

The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

 and younger sediments have accumulated in the deepest part of the basin near Vancouver

Vancouver, Washington

Vancouver is a city on the north bank of the Columbia River in the U.S. state of Washington. Incorporated in 1857, it is the fourth largest city in the state with a 2010 census population of 161,791 as of April 1, 2010...

. Most of the basin-fill material was carried in from the east by the Columbia River

Columbia River

The Columbia River is the largest river in the Pacific Northwest region of North America. The river rises in the Rocky Mountains of British Columbia, Canada, flows northwest and then south into the U.S. state of Washington, then turns west to form most of the border between Washington and the state...

.