Geoscience Reference
In-Depth Information
On the one hand, the absence of deep-ocean sediments on the continents suggests
stability. On the other, the fossil evidence indicates that the continents were once
connected. But over the twentieth century, additional evidence had accumulated
that “certain regions that were undoubtedly land long ago are now parts of the At-
lands?” (489).
He could see only two possible explanations of the broad facts of geology: con-
tinental drift or a horizontal stretching of the crust, after which isostasy caused the
thinned layers to sink (489). Wegener's ideas for the mechanism of drift he found
“hopelessly inadequate.” But then, showing just how different was his approach
fromthatofhisdogmaticAmericancolleagues,Holmeswrote:“Thereallyimport-
ant point is not so much to disprove Wegener's particular views as to decide from
the relevant evidence whether or not continental drift is a genuine variety of Earth
movement. Explanations may be safely left until we know with greater confidence
what it is that needs to be explained” (495).
“The choice,” Holmes wrote, “evidently lies between accepting continental drift
or postulating a giant land bridge across what is now 4,000 miles of ocean.” He
cited the geological evidence for drift, including the glaciation of the Southern
Hemisphere continents at the same time the Northern Hemisphere “enjoyed mild
or tropical climates” (499). Since these marked and contradictory differences from
the present climate patterns had taken place almost simultaneously, a worldwide
cooling orwarming trend could not explain them. Glacial striations and other evid-
ence showed that the ice had sometimes come from where now lies nothing but
open ocean. “With the continents in their present positions,” Holmes wrote, “such
a distribution of icesheets is hopelessly inexplicable” (501). But the facts became
explicable if “all the continents except Antarctica lay well to the south of the
present positions . . . grouped together around the South Pole” (503). Just as We-
gener had proposed.
The only argument Holmes could find against drift was that “it merely ex-
changes one embarrassing problem for another—the difficulty of explaining how
continental drift on so stupendous a scale could have been brought about” (504).
But, Holmes noted, if one rejects drift and the climatic evidence, one must then ac-
cept land bridges. “The continental drift solution has the advantage that it reduces
two baffling problems to one,” he wrote, referring to both the fossil and the climat-
ic evidence (504).
In My Geological Bones
