"... the river passes into the range of high Countrey at which place the rocks project into the river from the high clifts which is on the Lard. Side about 2/3 of the way across and those of the Stard. Side about the Same distance, the Countrey rises here about 200 feet above The water and is bordered with black rugid rocks ..."
[Clark, October 18, 1805]

"... The story of Wallula Gap begins back in the Miocene Epoch, about 17 million years ago. At that time, lava flows of basalt ran out of giant fissures in the Earth's crust east of here, near the Idaho-Oregon-Washington border. During basalt volcanism, the southern and western portions of the Columbia Plateau, including Wallula Gap, began to warp and fold under stresses deep in the Earth's crust. The bending of the ancient lava flows is clearly visible in the folded layers of basalt exposed in the steep walls of the gap.

Early in the history of folding, the ridgecrest here was slightly lower than elsewhere along the ridge. This caused first the ancient Salmon-Clearwater River (precursor to the Snake River), and then the Columbia River to flow across the ridge over this low point. As the ridge continued to bend upward, river erosion kept pace, and a water gap developed. Until about 10 million years ago only the ancestral Salmon-Clearwater River flowed through Wallula Gap. It wasn't until about 6 million years ago that the Columbia River joined in, where it has continued to flow ever since. Somewhere between 2 and 3 million years ago, the ancestral Salmon-Clearwater River captured the Snake River in the vicinity of Hell's Canyon, along the Idaho-Oregon border; this added significantly to the amount of water draining through the gap. ..."

"... the river passes into the range of high Countrey at which place the rocks project into the river from the high clifts which is on <both> the Lard. Side about 2/3 of the way across those of the Stard Side about the Same distance, the Countrey rises here about 200 feet above The water and is bordered wth black rugid rocks ..." [Clark, October 18, 1805]

Alexander Ross (Adventures of the First Settlers on the Oregon or Columbia River, published in 1849) journeyed upstream on the Columbia River in 1811 and reached the Wallula Gap and the Walla Walla River on August 12th.

"... On the 12th we left our camp early, and in a short time came to the colonnade rocks, which suddenly terminated in two huge bluffs, one on each side of the river, exactly opposite to each other, like monumental columns. The river between these bluffs lies right south and north. The banks of the river then become low with sand and gravel, and the plains open full to view again, particularly on the east side. Close under the right bluff issues the meandering Walla-Walla, a beautiful little river, lined with weeping willows. It takes its rise in the blue mountains already noticed. ..." [Ross, August 12, 1811]

In 1838 on the "Map of the United States Territory of Oregon West of the Rocky Mountains" the area of the Wallula Gap had it labeled "Gt. Bend.". Upstream was the "Walla Walla R." and "Ft. Nezperces". Downstream was the "Umatallow R.". The large basalt ridge across from the Walla Walla River was labeled "Volcanic M."

"... Twenty-five miles above Castle Rock stands the thriving little town of Umatilla, at the mouth of the river of the same name, and nine miles above is Windmill Rock. In ascending the river fifteen miles from this place, the land on either side rises to some fifteen hundred feet above the level of the river which occupies the entire bottom from rocks to rocks on either side; when the land suddenly drops from this high plain which extends from the Blue Mountains on the east to the Cascade range on the west, forming, as it were, a great inland dam across the Columbia River, fifteen hundred feet high at the place where the river has broken through the dam. As you pass out of this gap, in looking to the north and east, the eye rests upon another vast, high, rolling plain, in the southeastern part of which lies the beautiful valley of the Wallawalla. ...     The most of this vast, high, rolling plain, and especially the valleys, have more or less of alkali soil; the high plains are similar to those we have just passed, --- destitute of all kinds of timber, except at the foot of the mountains, and small patches of willow and cotton-wood, in some little nook or corner, near some spring or stream. ..." [Gray, 1870]

"Castle Rock" is the now historic location of Castle Rock, Oregon, with Umatilla and the Umatilla River located upstream. Nine miles above the Umatilla River is located Hat Rock and Boat Rock. Perhaps one of them is "Windmill Rock" ??? On the east side of the Wallula Gap is the Walla Walla River and the Blue Mountains, a spectacular view when heading east across the Gap.