Silurian Rocks at Arisaig, NS

Silurian rocks from Arisaig, Nova Scotia, Canada.

The Northumberland Strait shoreline of Arisaig Provincial Park in Nova Scotia, Canada, is described as one of the best sections of Silurian rock in the world. The strata are shales, sandstones, and siltstones from the Arisaig Group which was deposited in the early Silurian Period dating from about 443 to 424 million years ago.

I was fascinated by the way that some of the rocks were made up hundreds of extremely fine layers that were breaking up very easily. As far as I understand it, these darker shale layers were the result of deposits created in the coastal waters of the time by storm events rather than by tides or currents; and they are known as tempestites.

Hickman Hild and Barr (2015) say that the uninterrupted accumulation of fine-grained sediment during the Silurian Period, exposed here along a continuous 5 kilometre stretch, suggests that the area was tectonically quiet for at least 20 million years.

REFERENCES

Donohoe, H. V. Jnr, White, C. E., Raeside, R. P. and Fisher, B. E, (2005) Geological Highway Map of Nova Scotia, Third Edition. Atlantic Geoscience Society Special Publication #1.

Hickman Hild, M. and Barr, S. M. (2015) Geology of Nova Scotia, A Field Guide, Touring through time at 48 scenic sites, Boulder Publications, Portugal Cove-St. Philip’s, Newfoundland and Labrador. ISBN 978-1-927099-43-8, pp 50-53

Rocks at Moulin Huet – Part 1

General view looking east from the base of the steps at Moulin Huet Bay in Guernsey

Moulin Huet Bay lies on the edge of a plateau in southern Guernsey that is largely composed of Icart Gneiss. It forms part of the Southern Metamorphic Region in this Channel Island. Icart Gneiss is a pale grey, coarse-grained metamorphic rock containing large pinkish feldspar crystals in the midst of deformed masses of quartz, mica, and hornblende (de Pomerai and Robinson 1994) giving the rock a rather “squashed” appearance. It was originally an intrusion of granite, dating from around 2000 and 2500 million years ago, into even older rocks.

The colours and textures are extremely varied, depending sometimes on the angle and section viewed, the freshness of the exposure, degree of weathering, and number of encrusting organisms like lichens and algae. Some of these rock texture photographs are taken really close up so that you can see the individual crystals, especially the pinkish-orange feldspar. Others were taken at a greater distance showing the patterns of all the crystals within the matrix. The base of the cliffs was composed of this gneiss and so were the jagged outcrops on the beach (up to 5m in height) and the scattered boulders.

REFERENCE

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernsaise, ISBN 0 9518075 2 8.

Rip-Rap Rocks at Annapolis Royal

Boulders for sea defence along a shoreline

At Annapolis Royal in Nova Scotia a wooden boardwalk winds around the shoreline behind the town, giving unhindered views across the water of the Annapolis Basin to Granville Ferry and Port Royal on the opposite shore. The shore is bordered by salt marsh and mud which are vulnerable to erosion by the significant tides pumping in and out of the confined area of the Basin from the Bay of Fundy beyond. The natural strength of the ebb and flow are used in a tidal power electricity generating system at the nearby head of the basin.

The shore at the water’s edge is covered with dark angular blocks of stone which do not seem native to the location. They look like volcanic basalt, possibly from the Early Jurassic North Mountain Basalt (JN) dating from about 206 million years ago, which occurs naturally on the other side of the basin on the stretch of land that extends down to the Digby Neck and Brier Island. You can see this basalt outcropping at Delaps Cove which was described in an earlier post. It seems as if these basalt blocks at Annapolis Royal have been imported and positioned to protect the shore.

The basalt blocks are backed by salt marsh riddled with pools and runways. The raised pathway that passes around the back of theatre, and which provides such a pleasant walk past the dalek-like red and white lighthouse, is built on the saltmarsh. The boardwalk structure is protected on the seaward side by a line of sea-defence rip-rap. The rip rap is made up of a variety of rock types and includes boulders of local granite or granodiorite (Dg) which forms the bedrock that underlies the town. These are volcanic rocks from the South Mountain Batholith created during the Mid to Late Devonian Period from about 382 – 360 million years ago. Some of these granite boulders are shown in close up in the gallery below. These are the type of rocks mentioned in the earlier posts about Peggy’s Cove.

Mid to Late Triassic rocks of the Blomidon Formation (TJB) and Wolfville Formation (T) (dating from about 250 to 206 million years ago) are the foundation underlying the land on the Port Royal shore of the Annapolis Basin. However, as far as I could see, none of the siltstones, sandstones (including red sandstones), mudstones, or conglomerates from these formations had been used as rip-rap boulders here.

I did wonder if one or two of the rip-rap boulders fringing the boardwalk, that were clearly not granite, may have belonged to the Halifax Formation that underlies the landscape just to the south of Annapolis Royal. These rocks date from the Late Cambrian to Early Ordovician Periods 499 – 470 million years ago, and include slates and schists. Rocks of the Halifax Formation have featured in some earlier posts.

COPYRIGHT JESSICA WINDER 2013

All Rights Reserved

Rock around Chillagoe Part 3 – Balancing Rock

Balancing rocks occur in the rocky limestone outcrops, towers, and bluffs of the karst topography around Chillagoe, Queensland, Australia. These geological formations are created by the action of slightly acidic rainwater. This percolates into the cracks, crevices, and fissures of the stratified blocks of resistant stone exposed on the surface by the slow wearing away of overlying burden of sandstone.

The rainwater gradually dissolves the limestone and widens the cracks until individual blocks are formed and left balancing one upon another (Pictures 1 & 2). These features are common in the Chillagoe area but the one actually known as The Balancing Rock is a most spectacular giant monolith that looks as if it could topple over at any minute (Pictures 3 & 4).

A closer look at many of the limestone blocks reveals a surface texturing of parallel grooves or channels with sharp raised edges. These are also caused by slightly acid rainwater as it runs down the face of the stone. The type of erosional feature in these groovy rocks is called Rillenkarren and you can see various examples below in Pictures 5-10.



COPYRIGHT JESSICA WINDER 2012

All Rights Reserved