I visited the Racehorse Slide in Whhatcom County this week. The Racehorse Slide is a big slide that extended across the valley floor of the Nooksack River.
Lidar of racehorse slide
The first description of the slide may be attributed to the government land survey crews. I took a look through the field notes from the original government land survey that traversed this slide. Galbraith noted the piles of rocks and none of his survey witness tress were large.
Pringle, Schuster, and Logan (1998) took a crack at dating the slide via a log exposed in the Nooksack River bank below the slide. They got a date of 3,780 radiocarbon years ago. Given the location of the log this date can be viewed as a maximum date; the slide may be younger, and based on the appearance of the boulder field I suspect it is substantially younger.
Note lack of weathering rind on this coarse sandstone
Besides the rather unweathered appearance of the sandstone boulders, there are no large old stumps within the boulder field. Perhaps there are some elsewhere on the the slide. The lack of large stumps is by no means definitive regarding the slide age - it may simply mean that fire recurrence may have been frequent enough to prevent large tress from growing in this area. Early accounts of the area summarized by Cashman and Brunengo (2006) suggest that the area had been burned over in the late 1800s.