BOULDER-TOP BIRCH TREES AS A MEANS OF DATING A ROCK FALL EVENT: WISHBONE HILL, SOUTHCENTRAL ALASKA

Tree-ring science provides a means for dating some geological processes (e.g., mass movements) by using first year growth/pith as an event temporal marker. Accounting for the ecesis interval, the time between seed deposition and germination, provides a more representative date of the event than using the pith date alone. Wishbone Hill, Alaska contains evidence of a past rockfall event, denoting surface instability that could put the lives of those who recreate and reside in the area at stake. The source of the rockfall is an exposed syncline with a cliff face of conglomerate, which has sourced boulders that have accumulated in a boulder field at the base (mean boulder volume ~62m³). To estimate the age of the rockfall, 30 birch trees growing atop these boulders were sampled to maximize age. After mounting and polishing the samples they were dot-counted and ring widths were measured using Measure J2X software in order to generate a master chronology (1937-2017). In order to estimate the maximum age of the rockfall, pith/germination year for each birch sample must be either recorded or estimated. For samples that didn’t include pith (n=21), we had to use pith indicators to match existing rings to diagrams of corresponding ring widths, predicting approximate germination years for each sample. Then, for all samples we corrected for sampling height (mean=0.77m) using a low estimate growth rate (0.6m/yr). The oldest tree collected atop boulders at Wishbone Hill included pith and, with height correction, demonstrated a germination age of 1935; indicating the potential rockfall age. Because areas with evidence of rockfall events are known to have birch ecesis intervals of approximately five years (e.g., Van Der Burght et al., 2012), the youngest age of the rockfall event is ~1930. Risk of another destabilizing event may prompt those who use this area for recreational and residential purposes to reconsider future use.