"The backbone of the Blue Mountains begins in Grant County and runs northward through Baker, Union, Umatilla, and Wallowa counties in Oregon and into Walla Walla, Columbia, Garfield, and Asotin counties in Washington." [McArthur and McArthur, 2003, "Oregon Geographic Names"]

The Blue Mountains are mainly composed of sedimentary and metamorphic rocks and are generally divided into the Ochoco Mountains, the Aldrich Mountains, the Strawberry Range, Greenhorn Mountains, and Elkhorn Mountains (source: "Oregon Geographic Names", 2003), although local usage occasionally varies from these ranges (source: "geonames.usgs.gov" website, 2018).

"In every direction from the junction of those rivers the Countrey is one Continued plain low and rises from the water gradually, except a range of high Countrey which runs from S. W & N E and is on the opposit Side about 2 miles distant from the Collumbia and keeping its derection S W untill it joins a S W. range of mountains." [Clark, October 16, 1805]

And on October 17:

"I can proceive a range of mountains to the East which appears to bare N. & South distant about 50 or 60 miles." [Clark, October 17, 1805]

Other early references are listed in "Oregon Geographic Names" (2003, McArthur and McArthur):

"One of the first references to these mountains is by Gabriel Franchere, one of the Astorians. On arriving at the Walla Walla River, he wrote: "A range of mountains was visible to the S.E., about fifty or sixty miles off". He does not give the mountains a name. On July 9, 1811, David Thompson of the North West Company of Montreal referred to them as Shawpatin Mountains, but in his entry for August 8, 1811, he said: "Beginning of course to see the Blue Mountains between the Shawpatin and the Snake Indians." ... Alexander Ross, J.L. Townsend, David Douglas, Peter Skene Ogden, John Work, and other early travelers continued to use the name Blue Mountains."

In 1811, Alexander Ross wrote (published 1849):

"August 10, 1811:   On the 10th, at an early hour, we proceeded on our voyage, and met with no obstacle till the evening, when we arrived at the foot of a long and strong rapid Umatilla Rapids, now the location of McNary Dam], where we encamped near the mouth of a considerable river called Umatallow [Umatilla River], which enters the Columbia here. This river takes its rise in a long range of blue mountains, which runs nearly east and west, and forms the northern boundary of the great Snake nation."

"August 12, 1811:   Close under the right bluff [Wallula Gap] issues the meandering Walla-Walla [Walla Walla River], a beautiful little river, lined with weeping willows. It takes its rise in the blue mountains already noticed."

In 1830, John Work wrote:

August 22, 1830:   ... camp near the foot of the Blue Mountains on a branch of the Walla Walla."

In 1851, Rev. Gustavus Hines wrote:

"Descending these mountains to the east [Cascade Range], you come into the valleys, successively, through wich the river "De Shoots," John Day’s river, the Unatila, and the Walla-Walla flow, before emptying into the Columbia; and on the north side of the latter river, you come down into the valley of the north branch of the same river. On the north, this middle region is comparatively level, until you approach the northern ridge of the Blue mountains; but on the south side there are innumerable hills between the small rivers already mentioned, as also many plains of greater or less extent. As you approach the Blue mountains on the south, particularly on the Unatilla and Walla-Walla rivers, the hills disappear, and you find yourself passing over a beautiful and level country, about twenty-five or thirty miles broad, on the further borders of which rise with indescribable beauty and grandeur, that range which, from its azure-like appearance, has been called the "Blue Mountains." This valley, extending from the Cascade to the Blue mountains, is about one hundred and seventy-five miles broad, and the traveler in passing through it, meets with a continued succession of rocks, hills and plains of all dimensions, but generally he is well pleased with the face of the country."