Tuesday, May 3, 2016

Rock Fall and Root Throw

Assessing potential rock fall hazard on a slope meant taking a careful look at how the rock was weathering and joint patterns on cliff faces. Could this cliff spall off a large boulder? How much energy and momentum would a worse case rock fall have? What are the potential pathways of rock falls? What kind of material will the rocks encounter on the slope below? Will a rock fall maintain energy and speed while rolling or bouncing down the slope?

In assessing the cliff, I noted a rather loose boulder perched on the edge partially supported by a big leaf maple tree. My initial thought was that the tree had stopped a boulder from a cliff that was higher up the slope above the lower cliff area I was investigating.

In fact there was no cliff source area for the boulder. The slope above the cliff was a long flat area with absolutely no risk of rock fall. 

After a little thinking, I concluded that the precariously perched boulder got pulled up out of the ground some time in the past when a tree, now gone, (perhaps an earlier version of the big leaf maple) fell over. 

An example is this toppled tree that yanked a angular block of rock out of the ground and plopped it on the moss and brush. It is striking that such a modest looking tree has the root strength to pull out such a large rock from the bedrock below.

No comments: