Shell Rock, Oregon, Post Office was established April 14, 1873, in the vicinity of Shellrock Mountain. The Post Office was later moved across the Columbia into Washington Territory and re-named "Collins Landing". Briefly, in 1878, a second Shell Rock Post Office was established in Oregon. According to Oregon Geographic Names (McArthur and McArthur, 2003):
"Shell Rock post office was in operation again on the Oregon side between May 20 and August 19, 1878, with John Cates postmaster. Castes, of a well-known Wasco County family, operated a sawmill a couple of miles west of the present site of Wyeth. The post office was apparently reestablished to serve people connected with the Cates mill. It was about three miles west of Shellrock Mountain but named for that feature nevertheless."
I found some old foundations, way off the beaten path, in this area that could have belonged to the first Shellrock post office. Pure speculation, though.
Includes ... Shellrock Mountain ... Shell Rock, Oregon ... Mt. Talapus ... "The Dalles and Sandy Wagon Road" ...
http://columbiariverimages.com/Regions/Places/shellrock_mountain.html