football-sized nodules of bluestone
Thursday, September 4 2014
Down in the greenhouse, I directed my jackhammer at the undulating surface exposed yesterday in the bedrock. I found that the key to disassembling this layer was to attack the same geographic coordinates I'd attacked when I'd opened up the first hole in the hard layer of bluestone that had served as a impenetrable barrier since last autumn. There's a weakness at those coordinates that continues through the layer, and once I open up a hole to the next layer down centered on that spot, the layer is easily disassembled by cleaving off pieces from the edge of the hole until the hole is the size of the greater excavation hole itself. What is it about that coordinate that makes it so easy to attack? For whatever reason, the rock seems to contain vertical cleave planes there, and those planes contain intrusions of water, clay, and shale even if the bulk of the layer is crack-free bluestone. The layers of the rock itself lies very flat (sloping slightly downward at an angle of about 5:100 towards the northwest), though within those layers there are vertical structures in addition to the vertical cleavage planes. There are, for example, football-sized nodules of bluestone almost entirely surrounded by thin shells of shale. The rock was clearly deposited this way, though I cannot think of a mechanism that would make that happen.
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