Very little is known of the positions of the continents throughout most of the Precambrian Period. Sometime around 1,100 million years ago a large landmass called Laurasia began to form near the equator. Another landmass (Gondwanaland) formed 750 million years ago. It seems to have been located near the geographical south pole.

A fault across Nova Scotia from Cobequid Bay to Chedabucto Bay neatly divides the province into two geological zones which are fundamentally different from one another. The zone north of the Cobequid-Chedabucto Fault (Cape Breton and northern mainland Nova Scotia) is called the Avalon Zone. The Meguma Zone is the area south of the fault (southern Nova Scotia). Prior to the Devonian Period, the Avalon and Meguma zones developed in different areas and later came into contact along the Cobequid-Chedabucto Fault.

The Avalon and Meguma Zones are different because they belonged to different land masses and were widely separated from one another. The Avalon Zone was part of Laurasia, while the Meguma Zone belonged to Gondwanaland.

Rocks of Nova Scotia

The oldest rocks in Nova Scotia are the 1.4 billion year old gneiss, amphibolite and marble known as the Polletts Cove River Group and are found at the northeastern tip of the province. Not much is known about these rocks since the intense heat and pressure of metamorphism during several orogenies (mountain building periods) obscured their original compositions and structures. These old rocks were intruded by granite, diorite and anorthosite that were also later metamorphosed and deformed.

Two other groups of rocks, the Great Village River Gneiss and Mount Thom Complex in the Cobequid Highlands and the Kellys Mountain Gneiss and similar units in western Cape Breton Island, reveal little about their origins because of later metamorphism during mountain building. The Great Village River Gneiss, the Mt. Thom Complex and the Kellys Mountain Gneiss are between 1000 and 750 million years old.

Sometime between 850 and 700 million years ago (after a period of mountain building), erosion had reduced the high mountains to areas of lower relief. Here and in adjacent seas, sandstones, shales and limestones (George River Group, Gamble Brook Formation, McMillan Flowage Formation) were deposited unconformably on the older metamorphic rocks.

The end of the Precambrian era was a time of great volcanic activity, renewed mountain building and igneous intrusions. Rock units such as the Jeffers Group, Folly River Formation, Georgeville Group, Fourchu Group and similar units record the large volumes of basaltic and rhyolitic lava flows and tuffs that erupted on land and into ocean basins where muddy sediments were accumulating. The volcanism was a forerunner of the collision of two crustal plates that produced a period of mountain building known as the Avalonian Orogeny. Large amounts of dioritic and granitic magma were intruded into the folded and metamorphosed volcanic rocks. The volcanism and intrusions created numerous mineral deposits such as the copper-lead-zinc-gold deposit at Stirling and the copper and copper-molybdenum deposits at Coxheath, both on Cape Breton Island.

In southeastern Cape Breton Island, a thick sequence of brownish-red sandstones and rhyolitic lava flows of the Main-a-Dieu Group lies unconformably on top of the Late Precambrian Fourchu Group. Fossils have not been found as yet but the rocks are considered by some geologists to be transitional between Precambrian and Early Cambrian.

A lack of Precambrian rocks anywhere in the southern part of Nova Scotia is a significant difference between the southern and northern parts of the province. Some geologists believe that these old rocks do exist in southern Nova Scotia but are buried under the Meguma Group except where they are exposed in gneiss domes in the Liscomb Game Preserve.

Paleoenvironment

The Precambrian Period was a time of great volcanic activity, but it is mostly shrouded in mystery. We do know that the first hard-shelled creatures evolved approximately 900 million years ago. It wasn't until 650 mya that multi-celled animals were present on the planet.

The Precambrian rocks of Nova Scotia represent the remnants of volcanic islands adjacent to a deep sea trench. Such a geological environment is illustrated today by the Aleutian Islands west of Alaska.