Geologic maps indicate they're early volcanics from the Cascade range (about 30 million years old) that are harder to erode than the surrounding material. See the following links in addition to the one NotMaddie! (npub1afl…vtjm) shared:
https://pubs.oregon.gov/dogami/og/OBv20n06.pdf
https://gis.dogami.oregon.gov/maps/geologicmap/