Ralph Haugerud put to together a relative age map of some of the older landslides near the recent Oso/Hazel/Steelhead Landslide using LiDAR and cross-cutting relationships and smoothing (softening) of terrain.
Note his B age assignment to the very large landslide deposit I noted on a previous post aerial-history-and-lidar-of-stilliguamish-blocking-landslide. Besides the fact that the slide to the west of the Hazel slide is so large, note that it has thus far only been modestly eroded by the river since it was deposited suggesting a relatively young age.
Tom Schroeder sent me a mark up of witness trees from a Government Land Office survey done in 1890. These surveys provide a bit of a view of the past landscape.
a. line tree: redcedar 24"
b. line tree: hemlock 36"
c. quarter-corner witness trees: bearberry 10"; vine maple 5"
d. line tree: redcedar 60"
e. section corner witness trees: Douglas-fir 60"; Douglas-fir 72"; Douglas-fir 60"; hemlock 16"
f. line tree: hemlock 30"
g. line tree: hemlock 24"
h. steep bluff
i. quarter-corner witness trees: hemlock 9"; hemlock 5"
j. section corner witness trees: redcedar 18"; hemlock 16"; hemlock 5"; hemlock 15"
k. line tree: redcedar 80"
l. quarter-corner witness trees: spruce 36"; maple 24"
m. line tree: Douglas-fir 70"
n. line tree: redcedar 80"
o. line tree: redcedar 48"
p. section corner WT: hemlock 30"; hemlock 12"; redcedar 36"; maple 16"
q. quarter-corner WT: maple 7"; maple 30"
r. line tree: redcedar 30"
s. section corner: red alder 7"; red alder 6" (only 2 WT because very near left bank of river)
Perhaps nothing particularly definitive, but no huge trees on the slide deposit proper in the survey suggesting that this is not a very ancient feature. Of course there could be other reasons there are no huge trees. Some ground work looking for big stumps may provide some further clues or grabbing buried wood for carbon dates under the slide deposit along the incised river bank if it they can be found.
The take away for this area is that it is geologically dynamic and if a few landslide dates can be added the frequency of these dynamic events may be better ascertained.