Regional Geology of the Maritime Provinces

Once we have grasped the dynamic plate tectonic story told by the rocks beneath our feet, we can begin to understand one reason why the climate of the Maritimes has changed through geological time. Most of the crustal fragments in our region originated in high southern latitudes; came together as they moved northwards towards the equator, bearing tropical seas (think Windsor salt), then equatorial rainforests during the Coal Age (Joggins); suffered megamonsoons during the Permian (Brule and PEI) as drift continued into the northern sub-tropics; suffered sufficiently dry conditions to produce Sahara-like sand dunes in the Jurassic (Wasson Bluff); and were far enough north to be covered by glaciers that stretched to temperate northern latitudes in the Ice Age. This climatic story, long suggested by rock and fossil records, has been supported in recent decades by paleomagnetitic and plate tectonic studies.

But there is a second major source of climate change - global change on a planetary scale. As discussed in more detail in The Last Billion Years (Atlantic Geoscience Society, 2001), the Earth seems to have gone through "icehouse" and "greenhouse" phases, controlled perhaps in part by the distribution of continents and oceans. It comes as no surprise to most of us in Nova Scotia that the planet is currently in an icehouse phase. The Ice Age (the current or "recent" cold snap is conventionally spelled with initial capitals and given a definite article) began about 2 million years ago. But there were earlier ice ages, including devastating ones 600-700 million years ago that are now sometimes dubbed "Snowball Earth" episodes

Figure 13: Stages in the last retreat of glaciers from the Maritimes. From Atlantic Geoscience Society (2001)

There was also an ice age about 460 million years ago, an a series between about 250 and 350 million years ago - at the time of deposition of the Carboniferous rocks at Joggins - indeed, despite the fact that the rocks at Joggins record equatorial conditions, the world of the Coal Age forests was an icehouse one. We know this because glacial deposits ("tillites") have been discovered in Carboniferous successions in Africa. But deposits there are consolidated as hard rock, not soft, loose sediment as are the Ice Age "tills" that we see here in the Maritimes.

During the Ice Age (that is the recent ice age of the last 2 million years), there have been a number of advances and retreats of glaciers - termed glacials and interglacials. Stages of retreat of the most recent glacial are showin in Fig. 13. We are living during the most recent (Holocene) interglacial, which has now lasted about 10,000 years. The Holocene interglacial succeeded the latest (but probably not the last!), Wisconsinan, glacial. The mastodon found at Milford, Nova Scotia (and made famous at Mastodon Ridge at Stewiacke), lived about 90,000 years ago, during the previous, Sangamon, interglacial stage.

Many ancient events have left their mark on the modern-day topography of the Maritimes, but none have been as influential as the Wisconsinan glacial and its aftermath. Wisconsinan ice sheets held the region in their icy grip until about 16,000 years ago, when climatic warming triggered ice retreat rising sea level over a period of several thousand years. Truro at the head of the Bay of Fundy became ice free about 14,500 years ago, but between it and Dartmouth was a great lake that geologists now call Glacial Lake Shubenacadie. Trees appeared after 13,000 years ago and native people appeared about 12,500. Sea level, at 65 m below its present level, reached a nadir at 11,500 years ago. Bedford Basin was a lake until 9000 years ago. And Prince Edward Island became an island 7000 years ago.

Glaciers provide a great erosive force, acting like giant bulldozers. Evidence of glacial erosion include polished rock surfaces and striations, both of which can be seen at Peggys Cove, near Halifax. The Parrsboro Valley is an example of a glacially eroded valley; such valleys tend to be U-shaped rather than V-shaped - the latter form being typical or river-eroded valleys.

When they melt, glaciers tend to leave behind a thick blanket of mixed sediment, usually including lots of jumbled up mud and boulders, called "till". Glaciers often shape the till into elongate rounded hills called drumlins, of which perhaps the most dramatic in Nova Scotia is Citadel Hill in Halifax. Drumlins provide rare patches of fertile soil in Meguma country, so have always attracted settlers. (The explanation that we give for drumlins here is the conventional one, but there is currently exciting speculation that at least some drumlins might have been shaped by catastrophic, sub-ice floods - stay tuned ....) Till can also be referred to as "ground moraine". Moraine also accumulates at the sides of the glacier (lateral moraine) and in front of the glacier ("end moraine").

As demonstrated by till and moraine deposits, ice tends to leave unsorted sediment; water on the other hand usually leaves well-sorted deposits, and this difference allows geologists to identify the origin of material left by these two sediment-transporting agents. Glaciers, although made of ice, are associated with a lot of water: there is often water within or on top of a glacier, water beneath the ice, and a lot of water in front of the ice. Each of these situations leaves its calling card in terms of sediment. Water within or above the ice tends to leave a lumpy terrain after the ice melts, these so-called kame deposits being exemplified by the blueberry-growing area within the Parrsboro Valley north of Parrsboro. Water flowing in streams beneath the ice leaves long ridges of well-sorted sand deposits known as eskers. And outwash deposits are formed in front of glaciers can take the form of river or delta deposits: both Parrsboro and Five Islands are situated on former outwash deltas, where sediment was deposited on a former shoreline of the Bay of Fundy.

Everywhere you look in Nova Scotia, you see remnants of the last Ice Age. It has shaped the province and its inhabitants.