Plant fossils older than Devonian are rare. Soft-tissued plants were probably common before the Devonian (in the sea, for
example), but no fossil wood has been found in pre-Devonian rocks, although fossils of land plants in younger rocks are common. The plant fossils found in Devonian strata of the province are from some of the earliest land plants on earth.
The arkoses in the McAdam Lake Formation of Cape Breton also tell something about their genesis. Arkoses contain
abundant feldspar, which weathers rapidly to form clays. The presence of arkoses shows that conditions were arid, making the
weathering processes ineffective. (Remember that conditions here were tropical during Silurian time). The preservation of
feldspars in the arkose could also mean that the sediments were transported rapidly and deposited quickly before the feldspars
could be completely destroyed. The presence of conglomerates shows that the streams were fast enough to transport gravels, so it
can be surmised that transport and deposition were rapid. Rapid rivers require a steep gradient, so it can be inferred that the land
was hilly or even mountainous. The lacustrine deposits indicate that the rivers flowed into lakes that occupied valleys between
the hills.
In contrast, the Torbrook Formation of southern Nova Scotia has European-type fossils in sediments that were deposited in
bays or estuaries in the sea.
Southern Nova Scotia
In southern Nova Scotia events were entirely different from those north of the Glooscap Fault System and on Cape Breton
Island.
ORDOVICIAN PERIOD
As mentioned earlier, while the rocks of northern Nova Scotia and Cape Breton Island were being formed, southern Nova
Scotia was being developed on a completely different part of the globe.
South of the Glooscap Fault is a unique group of rocks, the Meguma Group, not recognized anywhere else on the whole
eastern side of North America.
The Meguma Group forms the bedrock to a land area of approximately 125 000 km2 (48 000 sq. mi.) (Figure 12) and may
have a thickness as great as 18 km (I I mi). The group is made up of metamorphosed sandstones and greywackes of the
Goldenville Formation that are in turn overlain by black to grey slates of the Halifax Formation. The two formations interfinger
in a complex manner. In individual exposures these rocks are characterized by sedimentary layers a few centimetres to a metre or
two thick; individual layers may be either discontinuous lenses or very extensive and uniform over a large area. The rocks show
many primary sedimentary features (scour and fill, flute casts, ripple marks and graded bedding) that were formed by turbidity
flows. Such flows develop when an accumulation of fresh sediments becomes unstable and slumps. Because the muddy (turbid)
water that results is denser than the surrounding clean water, it will flow down any slope on the sea bottom as a turbidity current.
This current, in consequence, moves the muds and sands into deeper water, where they are re-deposited. Because a flow may be
many kilometres wide and continue for a great distance, the layer of sediment deposited from it is uniform over a very large area.
The rocks so formed are called turbidites. The Meguma sediments were derived from repeated turbidity flows on submarine fans
at the edge of the
FIGURE 12.
23
Distribution of Meguma rocks (Cambro-Ordovician). Equivalent rocks are not recognized elsewhere in eastern
North America.
continental shelf, similar to the modern Laurentian fan at the southeast end of the Laurentian channel between Nova Scotia and
Newfoundland.
Sedimentary structures and other evidence within Meguma strata show that the original sediments were derived from a continent to the southeast, in the space now occupied by the Atlantic Ocean, and that the direction of the currents was to the northeast
(from Yarmouth towards Canso). Geological and geophysical evidence indicates that the former ocean basin that is now the Meguma
Group was peripheral to the northwest side of North Africa; that is, North Africa was the source of the sediment. This means
therefore, that the former ocean must have closed and caused a collision between Africa and America. (See Acadian Orogeny, page
25.) When the modern Atlantic Ocean began forming 190 million years later, the break occurred to the sou
theast of the old junction (the Glooscap Fault zone), leaving the Meguma rocks attached to America instead of their parent Africa.
LATE ORDOVICIAN TO EARLY DEVONIAN TIME Conformably overlying the Meguma Group rocks are Late Ordovician to
Silurian conglomerates, quartz sandstone and alkaline volcanic rocks (the latter containing high levels of potassium and sodium) and
related volcaniclastic sediments of the White Rock Formation. Compared with the Meguma sediments, the White Rock sediments
mark an abrupt and profound change from deepwater to shallow-water sedimentation. The conglomerates probably reflect erosion as
a result of uplift from the Taconian Orogeny. Another interpretation of the conglomerates is that they are glacial rills derived from an
ice age in the Saharan region of Africa. There was no folding of the Meguma strata before the deposition of the White Rock
sediments.