Nonconformity (unconformity between crystalline basement and sedimentary rock), SW Montana.

There is a lot to see in this photograph! The overlying Cambrian sandstone (Flathead Sandstone) is dipping some 20 degrees westward because of folding. It overlies Archean gneiss of SW Montana's basement complex. Note that there is a gravel lag deposit on the sole of the basal sandstone. There is also a deformed pegmatite dike that cuts through the gneiss, but does not cut into the sandstone. Given that the sandstone is Cambrian, it's some 520 million years old, and the gneiss, being Archean, is 2.5 billion years old --the woman has nearly 2 billion years of geologic time in between her two hands!


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