In the case of the Alps it seems natural enough that the crystalline masses
of Bohemia, the Black Forest and the central plateau of France should be firmer
than the more modern sedimentary deposits; but it is not so easy to understand
why the Mesozoic rocks of southern Germany resisted the folding, while those
of the Jura yielded. The Mesozoic limestone forms a very distinctive region
of the Balkans, notable for features such as the Kras.
The Quarternary Ice Ages had relatively little direct geologic influence
on the Balkans. No permanent ice caps existed, and there is little evidence
of extensive glaciation. Only the highest summits of Durmitor, Orjen, and
Prenj have glacial valleys and moraines as low as 600 metres. One geological
feature of great importance to the present-day landscape of the Dinarides
is that of the limestone mountains, often with their attendant faulting. They
are hard and slow to erode, and often persist as steep jagged escarpments,
through which steep-sided gorges and canyons are cleft by the rivers draining
the higher slopes. Limestone is a very porous rock, yet very hard and resistant
to erosion. During subsequent millennia these work deeper, leaving in their
wake enormous waterless caverns, sinkholes, and grottoes and forming underground
labyrinths of channels and shafts. The roofs of some of these caverns may
eventually fall in, to produce great perpendicular-sided gorges, exposing
the water to the surface once more. The magnificent gorges of many of the
Dinaric rivers, for example those of the Vrbas, Neretva, Tara, and Lim, are
justly famous. The partially submerged western Dinaric Alps form the numerous
islands and harbors along the Croatian coast.
The resisting mass is not necessarily at the surface. The most extensive
example of limestone mountains in Europe are those of the Kras (Classical
Karst) of the Dinaric Alps. Such is in outline the process by which the Alps
were elevated; but when the chain is examined in detail, it is found that
its history has not been uniform throughout; and it will be convenient, for
purposes of description, to divide it into three portions, which may be called
the Eastern Alps, the Swiss Alps, and the Western Alps.
The Eastern Alps consist of a central mass of crystalline and schistose rocks
flanked on each side by a zone of Mesozoic beds and on the north by an outer
band of Tertiary deposits. On the Italian side there is usually no zone of
folded Tertiaries and the Mesozoic band forms the southern border of the chain.
Each of these zones is folded within itself, and the folding is more intense
on the Bavarian side than on the Italian, the folds often leaning over towards
the north. The Tertiary zone of the northern border is of especial significance
and is remarkable for its extent and uniformity.
The Mesozoic belt of the Bavarian and Austrian Alps consists mainly of the
Triassic, Jurassic and Cretaceous (Mesozoic) beds playing a comparatively
subordinate part. But between the Trias of the Eastern Alps and the Triassic
of the region beyond the Alpine folds there is a striking contrast. North
of the Danube, in Germany as in England, red sandstones, shales and conglomerates
predominate, together with beds of gypsum and salt. The Trias of the Eastern
Alps, on the other hand, consists chiefly of great masses of limestone with
an abundant fauna, and is clearly of marine origin. The Jurassic and Cretaceous
beds also differ, though in a less degree, from those of northern Europe.
They consist largely of limestone; but marls and sandstones are by no means
rare, and there are considerable gaps in the succession indicating that the
region was not continuously beneath the sea.
Tithonian fossils, characteristic of southern Europe, occur in the upper
Jurassic, while the Gosau beds, belonging to the upper Cretaceous, contain
many of the forms of the Hippuritic Sea. Nevertheless, the difference between
the deposits on the two sides of the chain shows that the central ridge was
dry land during at least a part of the period.
The Triassic has almost disappeared, and what remains is not of the marine
type characteristic of the Eastern Alps but belongs rather to the continental
facies which occurs in Germany and France. Jurassic and Cretaceous beds form
the greater part of the Mesozoic band. On the southern side of the chain the
Mesozoic zone disappears entirely a little west of Lago Maggiore and the crystalline
rocks rise directly from the plain. The Eocene has altogether lost its independence
as a band and occurs only in patches within the Mesozoic zone. The latter,
on the other hand, assumes a greater importance and forms nearly the whole
of the subalpine ranges. It consists almost entirely of Jurassic and Cretaceous
beds, the Trias in these outer ranges being of very limited extent. But it
is divided longitudinally by a well-marked belt of stratified deposits, known
as the zone of the Brianconnais, composed chiefly of Carboniferous, Triassic
and Jurassic beds. The origin of the schistose rocks has long been under discussion,
and controversy has centred more particularly around the schistes lustres,
which are held by some to be of Triassic age and by others to be pre-Carboniferous
and even, perhaps, Archaean.
Upon the outer side of the arc the central zone of crystalline rocks is flanked
by Mesozoic and Tertiary belts; towards the west, indeed, the individuality
of these belts is lost, to a large extent, but the rocks remain. Upon the
inner side the Tertiary band is found only in the eastern part of the chain,
while towards the west, first the Tertiary and then the Mesozoic band disappears
against the modern deposits of the low land. The appearance is strongly suggestive
of faulting; and probably the southern margin of the chain lies buried beneath
the plain of northern Italy.
The chain of the Alps was not raised by a single movement nor in a single
geological period. Its growth was gradual and has not been uniform throughout.
In the Eastern Alps the central ridge seems to have existed at least as early
as Triassic times, but it has since been subject to several oscillations.
The most conspicuous folding, that of the Mesozoic and Tertiary belts, must
have occurred in Tertiary times, and it was not completed till the Miocene
period. The structure of the zones in the Bavarian Alps seems to suggest that
the chain grew outwards in successive stages, each stage being marked by the
formation of a boundary fault. A precisely similar structure is seen in the
Himalayas.