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and covered by Tertiary layers which form the continental
shelf and -slope. (Concerning these well-known results, we refer
to the survey in (79)).
This is a clear indication of a Mesozoic geosyncline in statu
nascendi, which ceased to grow when, towards the Cretaceous,
drainage was diverted westwards to a N-S depression and sea
lane right through the continent. It was now into this depres-
sion that the sediments were collected, ending with the Rocky
Mountains orogeny.
The present state along the east coast was further deter-
mined by the subsidence of the floor of the Atlantic at the
end of the Oligocene, cf. Chapter 7. In the Tertiary there was
no great drainage towards the east. Any major structure, like
a geosyncline was, therefore, not formed here in the Tertiary.
At present, almost the whole drainage of the U. S. is towards
the Gulf of Mexico, where a great amount of depressing sedi-
ments is being formed. But the closed conditions prevent long-
shore drifting on a large scale, as is evidenced by the form of
the Missisippi delta.
It is of interest to note that in case of the Rocky Mountains
geosyncline, an epicontinental lane was a sufficient condition
for the forming of a major orogenic cycle, when the drainage
was appropriate for the accumulation of huge amounts of
material; a main border between continent and deep sea is
thus not a necessary condition, although it is a favourable
one, if also the drainage system is such as to provide the
sediments.
The subsided abortive geosyncline, and the thick Mesozoic
strip of sediments along the east coast of the U. S. is a very
obvious continuation of a trend, all the time from the Gren-
ville, and through the Caledonian and Hercynian. All these
orogenic strips, and the Mesozoic strip of accumulation, are
parallel and have proceeded gradually ocean-ward with time.
Could this just have been an epicontinental sea lane, or is it
not rather obvious, taking a source of the sediments also into
account that this must have been a major border between con-
tinent and ocean for at least 1100 My? The importance of a