American hills seemed to float, almost ethereally, in the air above), was forced downward, sliding under the cliffs and hills like an envelope being pushed surreptitiously underneath a carpet. And second, while it was being slipped underneath, it dragged down with it all the sand and soft rock that had accumulated on top of it while it had been moving eastward from the center of the ocean. It had traveled a long way to the point of collisionâperhaps 5,000 milesâand it had taken a long time to arriveâperhaps 50 million years. The result was an ever-moving floor with an unimaginably large thickness of accumulated material on topâand some of it dipped under, carrying with it the floor material, while some stayed offshore, like surplus froth scraped off a cappuccino. It remained like a great barrier of islands, well to the west of the place where one plate was sliding under the other.
Thus a basin was created between the hills at the point of collision and the hills of the offshore islandsâwith the former (to reiterate) being a complicated and multilayered arrangement of ocean floor together with mixed-up and younger sediments, the latter being rather more entirely youngish mixed-up sediments with little or no ocean floor anywhere around, other than deep, deep down below. And over the next many millions of years, more sediment accumulated in the basin, the makeup of which was determined by how it was accumulating (whether in silt-rich rivers, on some sandy shoreline, by settling on the bottoms of deep oceans, or born from onetime marshes or long-buried sand dunes) and what the weather was like while it was doing so (warm and humid or freezing and dry).
There must have been one period, about 40 million years ago, when huge ferns and soft trees crashed, dying, into warm and fetid swamps. The layers of material in the swamps were then compressed and buried and heated, and after time and maturation they produced the coal of the Domengine Formation for which the miners hunted so assiduously in the 1860s. But there were other periods when there were deep blue seas instead of botanically rich swamplands, and they were alive with shellfish, sharks, and other noble creatures; and there were other more recent times when mastodons and saber-toothed cats trekked over windy grasslands and left their bodies to rot and become skeletons in forests of trees that seem not dissimilar to those growing in California today.
And then, 20 million years or so ago, the oceanic plate suddenly changed direction, for reasons that will be made clear (or as clear as the science will allow, since much remains unexplained) in a later chapter. Instead of pushing eastward, to smash head-on into the cliffs and hills of North America, the oceanic material began to move northwestward , proceeding smartly up alongside the coast, scraping past it instead of plunging underneath it.
The connection with San Francisco now becomes clear, for this new movement is exactly the same movement between plates that would go on to produce the cracks in the earthâs surface that nowadays trigger the myriad earthquakes that occur farther to the west. At Mount Diablo, though, the movement did something rather different: It caused all the material that had plunged below the North American Plate to be ripped northward, bringing it hard up against the newer sediments that had been accumulating behind where it had been diving downward. The newly arrived material began to wrap itself around the older crustal and downthrust rocks, as if it were an immense shell of pastry. In time it buried the older material entirely and made it more or less invisible, coated in a thick covering.
And there this material would have remained, except that about 4 million years ago, for reasons that will also be explained later, the whole mixture was dramatically folded upward. The old, hard rockswere in the center of the fold; the new, young, soft sediments were on the outside.
Thus a basin was created between the hills at the point of collision and the hills of the offshore islandsâwith the former (to reiterate) being a complicated and multilayered arrangement of ocean floor together with mixed-up and younger sediments, the latter being rather more entirely youngish mixed-up sediments with little or no ocean floor anywhere around, other than deep, deep down below. And over the next many millions of years, more sediment accumulated in the basin, the makeup of which was determined by how it was accumulating (whether in silt-rich rivers, on some sandy shoreline, by settling on the bottoms of deep oceans, or born from onetime marshes or long-buried sand dunes) and what the weather was like while it was doing so (warm and humid or freezing and dry).
There must have been one period, about 40 million years ago, when huge ferns and soft trees crashed, dying, into warm and fetid swamps. The layers of material in the swamps were then compressed and buried and heated, and after time and maturation they produced the coal of the Domengine Formation for which the miners hunted so assiduously in the 1860s. But there were other periods when there were deep blue seas instead of botanically rich swamplands, and they were alive with shellfish, sharks, and other noble creatures; and there were other more recent times when mastodons and saber-toothed cats trekked over windy grasslands and left their bodies to rot and become skeletons in forests of trees that seem not dissimilar to those growing in California today.
And then, 20 million years or so ago, the oceanic plate suddenly changed direction, for reasons that will be made clear (or as clear as the science will allow, since much remains unexplained) in a later chapter. Instead of pushing eastward, to smash head-on into the cliffs and hills of North America, the oceanic material began to move northwestward , proceeding smartly up alongside the coast, scraping past it instead of plunging underneath it.
The connection with San Francisco now becomes clear, for this new movement is exactly the same movement between plates that would go on to produce the cracks in the earthâs surface that nowadays trigger the myriad earthquakes that occur farther to the west. At Mount Diablo, though, the movement did something rather different: It caused all the material that had plunged below the North American Plate to be ripped northward, bringing it hard up against the newer sediments that had been accumulating behind where it had been diving downward. The newly arrived material began to wrap itself around the older crustal and downthrust rocks, as if it were an immense shell of pastry. In time it buried the older material entirely and made it more or less invisible, coated in a thick covering.
And there this material would have remained, except that about 4 million years ago, for reasons that will also be explained later, the whole mixture was dramatically folded upward. The old, hard rockswere in the center of the fold; the new, young, soft sediments were on the outside.
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