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Science Frontiers ONLINE No. 11: Summer 1980 |
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Orphans Of The Wild West
North of San Francisco, all along the Oregon and Washington coasts, the geologically oriented traveller will discover many huge boulders, mostly 10-20 m across, but some 100 m in size. Their constitution varies, but many are coarse-grained basalts that appear to have spent much of their lives at least 30-40 km underground. These boulders are "erratics" in the sense that no one has found surface outcrops that might have given them birth. So, where did they come from? But origin is only part of the problem. The presumable non-glacial erratics occur in a geologically confused area that seems to be upsidedown time-wise according to the few fossils that have been found.
One theory is that the erratics were long ago carried to great depths by the conveyor-belt layers that slide eastward and downward under the U.S. Pacific Coast. Later, geological pressures squeezed the rock containing the erratics back to the surface like toothpaste. In the last phase, the matrix rock was eroded away leaving the erratics orphans.
(Wood, Robert Muir; "Orphans of the Wild West," New Scientist, 85:466, 1980.)
Comment. Note that this complex scenario is dictated by the dogmas of continental drift and the geological time scale.