Around 1.2 billion years ago, plate tectonic motion pushed together fragments of continental crust which created the supercontinent Rodinia ("homeland" in Russian).

Rodinia was the dominant landmass on Earth for around 350 million years. The exact size and configuration of Rodinia is not known and is debated by scientists.. It is believed that what is now North America's east coast was adjacent to western South America. The west coast of North America was next to what is now Australia and Antarctica.

During this time the area that would become Washington was a vast floodplain that accumulated thick sequences of sand and silt. The rust red colored landscape would have been devoid of plants as life on Earth had not progressed beyond single-celled algae. The atmosphere would have only had 5% of the oxygen it now holds and lacked an ozone layer. The only single ocean of earth would have been spawning violent storms over the water. There would have been no protection for the landscape from the forces of water. Great floods would have been common on Rodina.

Under these desolate and violent conditions, the first rocks of Washington formed. Between 700 million and 1.5 billion years ago a distinct group of sedimentary rocks accumulated in a giant basin over part of what is now Washington, as well as Alberta, British Columbia, Montana, and Idaho. Scientists named these rocks the "Belt Supergroup" after a nearby town. The Belt Supergroup rocks are multicolored sandstone, siltstone and limestone. They are noted for their beautiful preservation of sedimentary features such as mud cracks, ripple marks and abundant "stromatolites." Stromatolites are cabbage shaped fossils left by the action of blue-green algae know as cyanobacteria. In Washington, the Belt rocks are exposed extensively in the eastern Okanogan Highlands. The Belt Group also makes up most of the spectacular mountains in Glacier National Park in Montana. The Belt rocks are remarkably thick. They accumulated to perhaps 50,000 feet near the Washington-Idaho line, and thin eastward toward central Montana. The rupture that split Rodinia abruptly truncated the basin in which the Belt sediments accumulated. The missing western margin of the Belt basin is now thought to be part of either Australia or Asia.

References

Read more about Washington's Geological past at the Burke Museum