Some things are just infuriatingly difficult to pin down in geology. For instance, just how deep was our pebble sea, the Silurian sea of the Welsh Basin at the spot that became, some 400 million years later, the beach beneath our feet? Well, one can estimate some kind of minimum depth. It was deeper than the depth to which waves and tides can leave a trace on a sea floor, because no traces of these phenomena have been found in the pebble stuff or—rather more convincingly as evidence—in any of the strata of those Welsh cliffs from which the pebble could have been derived. As a rule of thumb, that means that the sea was more than a couple of hundred metres deep, that being the depth to which the very biggest waves of the very biggest storms on a wide open sea can stir the sea floor. Now, if strata have been deposited above that level, then one can make some reasonable estimates of ancient water depth. Thus, if one finds fossilized beach-strata, that is an obvious signal that those rocks were formed virtually at sea level. And below that, we can make a distinction between those shallow sea floors that are stirred pretty well all the time, even by the small waves of a fair-weather day (on this kind of sea floor, mud is winnowed away, and only sand and pebbles can settle); and those deeper sea floors only affected by the biggest storms (where thick muddy layers can settle in between major storms that may have been a decade—or a century—apart). But below even that? It is, in practical terms, hard to tell from the rock strata whether the ancient sea floor on which they were laid down was 300m or 3000m deep, or perhaps even more. So it is with the pebble rock. This Welsh sea floor was deep in general terms, but its precise depth remains a mystery—working out even a reasonably imprecise depth remains as a puzzle for future generations of geologists to solve.
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