DENDROCHRONOLOGICAL CONSTRAINTS ON COASTAL EROSION RATES AT ERIE BLUFFS STATE PARK, NW PA

Erosion threatens bluff stability along the Lake Erie shoreline and poses a lingering threat to neighboring communities and recourses. Consequently, accurate constraints on erosion rates is critical for shoreline conservation. A section of coastline of particular interest is at Erie Bluffs State Park (EBSP) near Girard Township, PA. The bluffs at EBSP are ~20m tall and are composed of fine glacial sands that reside on laminated glacial clays (Babcock et al., 1998). EBSP bluff faces are characterized by steep-sloped, scalloped edges that transition downslope into convex surfaces suggesting the dominant erosion process near the top of the section is slumping. In an attempt to help constrain erosion rates along EBSP, we report here on our attempts to use tree rings to date the timing of two slump events (two slump blocks - ~42.01°N 80.37°W and ~42.02° N, 80.38° W) at EBSP. To date these events, we collected 8 cores from 8 trees across the 2 sites (4 cores per site); cores were collected from vertical trees (i.e., trees that clearly post-dated slumping) along the head scarp as well as the top of each block. Cored trees included beech, red maple and birch. The oldest trees were in the head scarp and had an average age of ~45 years (max tree age is 49 years and the youngest is ~ 40 years; pith was not reached) suggesting the slump events occurred prior to the mid to early 1970's. Trees exhibit a gradual age decrease (as determined from cores and as derived from circumference measurements) downslope suggesting the downslope surfaces are re-worked more frequently than the upper slopes. This is consistent with the youngest trees occurring in clay materials exhibiting evidence of multiple periods of Earth flow. Triggers for Earth flow likely include ground saturation and undercutting of the bluff face by wave action (e.g., Cross et al., 2007); there is a notable absence of mass wasted bluff deposits immediately along the lake shore suggesting lake activity may also remove the deposits after failure. In such a scenario, it seems likely that further undercutting may trigger additional Earth flow. Given the younger tree ages at the bluff base, perhaps the Earth flow deposits serve to temporarily ‘armor’ the bluffs from additional slumping events. Future work will incorporate historical imagery, ArcGIS, and field data to help further constrain erosion here.