Welcome to Manuels River!

Manuels River Natural Heritage Society

Development of the Avalon Peninsula, as we now know it, has been a long and complex process, involving continental drift, the destruction and creation of oceans, volcanism, faulting, erosion and eventually, glaciations. The resultant rocks have been divided into groups and formations, based upon their type and/or interpretations of their origins.

The oldest rocks on the Avalon Peninsula, and along the Manuels River, are those of the Harbour Main Group. These are primarily volcanic, and consist of rhyolites and basalts: two rock types which are produced by magmas crystallizing at the earth's surface and which are differentiated by different chemical compositions. Also present are pyroclastic rocks, which are like sedimentary rocks formed, however, from volcanic ash and ejected fragments. The volcanoes responsible for the rocks of the Harbour Main Group formed approximately 650 to 600 million years ago (during the Precambrian), probably as part of a chain of volcanic islands near the edge of a landmass attached to present day Northern Africa. Volcanic eruptions also occurred underwater in the surrounding seas.

Late in the formation of the volcanic mountains, around 615 million years ago, a large body of molten rock, or magma, melted its way up into the volcanic rocks and sediments in the region. The magma did not reach the surface, but slowly cooled and crystallized at depth to form a granite. This rock, the Holyrood Granite, gradually became exposed in places as overlying volcanics and sediments were eroded and now forms much of the geology to the southwest of the drainage basin. However, it has a limited occurrence within the Manuels River system, occurring as a small outcrop, about 300 to 400 meters in diameter, immediately upstream of the Conception Bay Highway bridge. There are bits of Harbour Main Group volcanic rocks evident within the granite. These became caught up in the magma as it intruded upwards through the rock and remained in place once the magma had cooled and crystallized. There is also a small strip of the Holyrood Granite on the southwestern edge of the watershed.

Continental drift had produced a small ocean about 900 million years ago, which was closing as continental plates moved back together. During this period, the western part of Newfoundland (the Great Northern Peninsula) was situated on another landmass, while central Newfoundland, as we know it, had not yet formed. The "assembly" of Newfoundland would not occur until hundreds of millions of years later, with another continental collision. Within the Manuels River system, rocks of the Harbour Main Group form much of the river bed upstream of the Conception Bay Highway bridge, but are terminated by the Topsail Fault further inland.

The next oldest rocks in the region are those of the Precambrian Conception Group. These rocks, which primarily occur inland of the Topsail Fault are sediments which were formed by the erosion of the volcanoes and deposition of the resultant sediment in the ocean nearby. They are around 560 million years old. Much of the sediment was carried into deep water by "turbidity currents"; loosely comparable to an underwater landslide or avalanche.

There are no known fossils within the sediments of the Conception Group that occur within the watershed region. However, the youngest rocks, at the top of the Conception Group, display spectacular fossils at the ecological reserve at Mistaken Point, on the southeastern tip of the Avalon Peninsula. Rocks of the Conception Group have largely been eroded from the portion of the Manuels River drainage area lying west of the Topsail Fault.

Following the deposition of the Conception Group sediments, seas in the region gradually became in filled with sediments. Rocks of the St. John's Group record the transition from deep ocean to an environment where rivers built large deltas of sand out into the sea. These features covered over older, deeper water sediments. Rocks of the Signal Hill Group originated from deposition of sands and gravels on land, and were primarily deposited by rivers and floods. Neither the St. John's nor the Signal Hill Groups are present within the Manuels River drainage basin, although they were originally deposited over much of the present day Avalon Peninsula. They were gradually eroded, along with the sediments of the Conception Group, after the centre of the Avalon Peninsula was pushed upwards by the movement of continental plates about 550 million years ago. Numerous folds, along with faults, were also developed.

During the period of time discussed so far, 750 to 600 million years ago, the ocean in the vicinity of the study area was in the process of closing. This was reversed about 600 million years ago when a major new ocean, known as Iapetus, began to form. The Avalon Peninsula was on the edge of this new ocean as continental plates began to drift apart. As this progressed, sediments were again deposited upon areas of the peninsula which had previously been eroded. This scenario produces what is termed an "unconformity"; when the formation of rocks is interrupted by either a period of inactivity or erosion.

One example, perhaps the best, of this particular unconformity on the Avalon Peninsula occurs within the Manuels River system. Near the Conception Bay Highway bridge, a 5 to 6 meter thick conglomerate directly overlies the eroded Holyrood Granite. This conglomerate is approximately 540 million years old (of Cambrian age) and probably represents an ancient beach, developed as the new ocean (Iapetus) was opening. The pebbles and cobbles which make up this sediment are very well rounded, and look like those that occur on any of the local beaches.

The conglomerate dips approximately 10 degrees or so downstream. This sequence, along with the overlying rocks, was originally flat-lying, but has been tilted by folding about 200 million years later. Above the conglomerate are two groups of Cambrian shales (560 to 500 million years old) which comprise the geology of the lower part of the Manuels River. These rocks were formed from shallow marine sediments deposited as Iapetus developed and it is here that the trilobite fossils are found.

The Adeytown Group is the older of the two and occurs as an approximately 100 meter thick sequence of greenish-grey shales directly above the conglomerate. As mentioned, the shales dip gently downstream (the thickness of the sequence is measured as though the rocks were in their originally flat orientation). There are also thin beds of limestone present, along with some reddish shales, representing changing sediment sources and conditions at the seafloor. There are a number of shale beds which contain nodules, or centimeter-sized irregular balls, of manganese, occurring about 20 m above the bottom of the shale sequence. This is a mineral which would have formed during extended geological periods when no sedimentation was occurring.

The transition to overlying grayish-black shales of the Harcourt Group occurs near the location of the fossil beds behind the Villa Nova School and is marked by a thin, 3 centimeter thick, band of whitish clay. This band probably originated as fine grained ash from a volcanic eruption. The change to black shales at this point probably reflects a change to a less oxygen rich seafloor environment and/or an increase in water depth, and may have resulted in a change in species of trilobites present. Pyrite, or "fool's gold", is a common mineral within the shales of the Harcourt Group, and forms from iron and sulphides within the black shales (pyrite = Fes2). This is another indication of a low oxygen environment of deposition. Some limestone also occurs in limited, irregular zones. About 200 m of Harcourt Group shales are exposed along the Manuels River, before reaching the present day coastline. Sandstones become increasingly common towards the coast. The Harcourt Group has been divided into two formations, one of which is the Manuels River Formation; a name chosen to reflect the locality where it is best observed.

Offshore, a 900 meter thick sequence of shales, with siltstones and sandstones, continues beneath Conception Bay. Overlying the Cambrian rocks is a 1.2 kilometers thick sequence of Ordovician age (500 to 440 million years ago) which for the most part consists of iron rich sandstones. These rocks form Bell, Kelly's and Little Bell Islands and, like the Cambrian rocks on the shore of Conception Bay, were formed in a shallow marine environment. On Bell Island, fossilized trilobite tracks are commonly found. The iron rich (hematite) beds were mined in decades past.

The opening of Iapetus, the precursor of the Atlantic Ocean, which resulted in the deposition of the Cambrian and Ordovician marine sediments, continued from about 600 to 450 million years ago. At this time, the continental plates began to move together again. By 390 million years ago, Iapetus had disappeared, and the geological sections which today form Newfoundland had been "assembled".

The Northern Peninsula had previously been on the far side of Iapetus, while Central Newfoundland was created from the old deep ocean bedrock of Iapetus that had been pushed up in the continental collision. The rocks of the Avalon Peninsula experienced some disruption during this episode, causing, amongst other things, the Cambrian and Ordovician rocks to dip to the northwest.

The opening of the present day Atlantic Ocean began approximately 150 million years ago, at which time the continents split apart immediately to the east of the Avalon Peninsula. This, of course, left the Avalon Peninsula attached to Newfoundland and North America. The present day geology of the Avalon Peninsula originates from this long, complex and convoluted history, but also reflects the millions of years of subsequent erosion (including that of glaciations) that have occurred since.

None of the sediments which may have been deposited since about 350 million years ago remain upon the Avalon Peninsula (except for very recent sediments from the last glaciation). Therefore, no possible evidence of dinosaurs (or the like) can be found. Sediments of suitable age (Cretaceous to Jurassic) do occur at depth beneath the seafloor on the Grand Banks of Newfoundland, and it is in these rocks that oil and gas reservoirs are presently being discovered.

As is the case for all of Newfoundland and Labrador, the Avalon Peninsula has undergone one, if not many, glaciations (or "ice ages"). The number is uncertain because every glaciation that occurs tends to remove (erode) all evidence of past glaciations. Evidence from elsewhere in Newfoundland and the world does indicate that glaciations have been occurring on a regular basis for millions of years.

The current belief is that during the last glaciation, from about 25,000 to 10,000 years ago, the Avalon Peninsula was covered by an ice cap. An ice cap may be loosely viewed as one large glacier. The rest of Newfoundland was also ice covered at this time, but by separate ice caps. The ice was centered and thickest near the middle of the Avalon Peninsula (i.e.; near Whitbourne), and spread down the centerline of the secondary peninsulas (such as the St. John's Peninsula).

In the Manuels River area, the ice inland was probably on the order of 500 to 700 meters thick, and thinned towards the coast. There may also have been ice within Conception Bay. During the initial advance of the ice, pre-existing soil and rocks were eroded. As the ice melted, much of this sediment which the ice had picked up was washed down by water from melting ice to gather along the present day shoreline of Conception Bay. This sediment is quite apparent at many places along the shore today, as a poorly sorted mixture of boulders, pebbles, sand and silt. The rocky beaches of the shore near the Manuels River probably formed from pebbles and cobbles washed out of these sediments by waves in the last 10,000 years. At the base of the waterfalls immediately downstream from the Conception Bay Highway bridge a large collection of boulders reflects transport by glacial ice.

Inland from the coast, the glacial sediment becomes less abundant. It is usually less than 5 meters thick and occurs in depressions in the bedrock. A series of sediment ridges created by the retreating glacier occurs near the bank of the river, from approximately 1.5 to 4 kilometers upstream from the highway bridge.

Manuels River, along with surrounding rivers, probably acted as a path for inland ice to move down to the coast. This would have caused erosion and deepening of the river valley, especially where the bedrock consisted of relatively soft sediments (i.e.; compared to the volcanic rocks). As well, it probably acted as a drainage channel for water when the ice was melting, allowing for even more erosion. The Cambrian shales directly downstream from the highway bridge have been most heavily eroded of the rocks along the river system, producing a steep sided valley and a rapid change in elevation. The sea level may have been lower than at present during some point in the deglaciation, so that the flow of water at the present river mouth was more forceful, and eroded the wide estuary we see today.

The mouth of the Manuels River consists of a broad estuary, separated from Conception Bay by a gravel barrier beach or barachois. The water in the estuary is brackish, reflecting an elevation near that of sea level. At high tide, the sea can be seen to wash back into the estuary through a small break in the barachois. This may not have occurred during earlier times when the flow of water in Manuels River was substantially greater than at present (i.e.; prior to diverting much of the headwaters of the system). It is expected that the water levels in the estuary prior to the headwaters diversion of the Manuels River were a little higher than at present and certainly the water was considerably less saline. This would obviously have depended, as it does today upon the changing water flow rates during the run of any given year. Fresh water reaches the sea through the opening in the barrier beach berm, as well as by filtering through the coarse sediment and cobble which forms the beach.

It is likely that the estuary acts as a sediment settling pond, especially since the water flows have been diminished by headwaters diversions and during times of seasonally low river flow. Sediment which has been transported downstream by relatively fast moving water is given a chance to settle out once the water reaches the sluggish environment of the estuary. Combined with this, salt (or brackish) water generally causes clay sized grains to clump together and settle out, when they might not otherwise. Local residents confirm that the bottom of the estuary is muddy and that the mud is fairly thick. This poses a potential problem for recreational use of the estuary, in that swimmers or boaters may get stuck in this mud while jumping in or during a capsizing.

The barrier beach across the mouth of the estuary is similar to those which commonly occur along the eastern shore of Conception Bay and elsewhere on the island of Newfoundland. Such features develop as a result of ocean currents and storm waves. The pebbles and cobbles which make up the beach probably originated from glacial sediments along the shore. Waves have eroded the fine sediment from between the coarse sediment and carried it into deeper water offshore. Waves and currents flowing along the shore from the southwest, driven partly by water from the Labrador Current, have transported the pebbles and cobbles to build the linear barrier beaches that occur in association with the Manuels River estuary, along with Chamberlains Pond and Topsail Pond. Periodic storms continue to alter the shape of these features, both removing and replacing sediment from the beaches.