Pebbles from St. Martins, New Brunswick

Red, green and patterned pebbles in a dish

St Martins in New Brunswick, on the Bay of Fundy coast in Canada, is famous for its red Triassic cliffs. These have been eroded by the sea to form large caves in which seals like to rest. The adjacent long beach is composed of millions of pebbles or beach stones from not only the nearby Triassic strata but also brought down from far and wide by the massive ice sheets that once covered the land mass. There is an amazing variety of rock types represented by the pebbles, with red and green colours most noticeable, and many textures and patterns exhibited.

An assortment of patterned and coloured pebbles

Red, green and patterned pebbles in a dish

COPYRIGHT JESSICA WINDER 2014

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Ringstead Bay Rock Textures & Patterns – Part 3

Another gallery of close-up photographs showing natural colour, textures and patterns in an assortment of rocks on the beach at Ringstead in Dorset, England, illustrating the great variety in the geology of this stretch of World Heritage Coast.

COPYRIGHT JESSICA WINDER 2014

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Pebbles at Saints Rest Beach

Natural patterns and colours in pebbles of mostly igneous rocks like granite on the shore at Saints Rest Beach, Irving Nature Park, New Brunswick, Canada.

The pebble beach at Saints Rest extends for one kilometre between the mainland and the peninsula that is covered by Irving Nature Park near Saint John, New Brunswick, Canada. The gravel and sand of which it is composed are precariously held together by the roots of marram grass. The beach forms a barrier between the sea and the salt marshes and river valley beyond.

The shore is covered by an amazing variety of pebbles of different geological origins, exhibiting a wide range of natural colours and patterns. Many or most of the smooth pebbles are derived from igneous rocks like granite found in the deposits of glacial moraine that covers much of the surface area in the region – rocks that were plucked up some distance away when the massive ice sheets passed over the native bedrocks, and then were eventually released in the thick superficial layers of sediment as the ice sheets melted.

Some of the other pebbles on the beach must have originated from local bedrocks that lie beneath the loosely compacted glacial moraine deposits covering the river valley, Irving Nature Park and Sheldon Point. The solid bedrock here belongs to the Taylor Island Formation, composed of Precambrian volcanic rocks (ZCTIvs). This underlying solid geology is visible in outcrops around the coastline of the nature park as well as on the headlands to the east and the west of Saints Rest Beach.

Natural patterns and colours in pebbles of mostly igneous rocks like granite on the shore at Saints Rest Beach, Irving Nature Park, New Brunswick, Canada.

Natural patterns and colours in pebbles of mostly igneous rocks like granite on the shore at Saints Rest Beach, Irving Nature Park, New Brunswick, Canada.

Natural patterns and colours in pebbles of mostly igneous rocks like granite on the shore at Saints Rest Beach, Irving Nature Park, New Brunswick, Canada.

Natural patterns and colours in pebbles of mostly igneous rocks like granite on the shore at Saints Rest Beach, Irving Nature Park, New Brunswick, Canada.

Natural patterns and colours in pebbles of mostly igneous rocks like granite on the shore at Saints Rest Beach, Irving Nature Park, New Brunswick, Canada.

View of Saints Rest Beach, New Brunswick, looking west.

COPYRIGHT JESSICA WINDER 2013

All rights reserved

Fault Gully Rocks at Mewslade Bay

Fault breccia with calcite matrix and Old Red Sandstone

As you arrive at the narrow gully that forms the entrance to Mewslade Bay, you cannot help but notice just how different the rocks are from those that outcrop on the hills around. Up on Thurba Head to the left and to the high cliffs and outcrops on the right, thick grey layers of High Tor Limestone from the Carboniferous period are clearly visible. However, down in the gully, there is nothing but a jumble of assorted broken rocks of all kinds of origin, which in some places have been cemented together with white crystalline calcite.

Fault breccia either side the gully to the bay

The reason for the unusual geology at this point is an ancient rupturing and tearing of the rocks – faulting. The steep-sided dry valley that leads from Pitton village down to Mewslade Bay follows the line of a fault. In fact, two faults converge here.  The fault line itself extends inland and North-north-east across first the High Tor Limestone which appears as cliffs along the South Gower shore in this locality, then passes across Hunts Bay Oolite (the next in the succession of Carboniferous Limestone strata), and possibly other rocks in the series such as Gully Oolite. Further north, around and beyond Pitton and Middleton, superficial drift deposits of glacial till, like gravelly clay, mostly cover and obscure the solid geology below. Beneath this layer of till deposit lies the solid Old Red Devonian Sandstone rock.

Fault gully entrance to Mewslade Bay

Faulting occurs as brittle rocks crack when they can no longer bend and fold to accommodate earth movements. Once the rocks break, there can be major movements of the strata on each side of the crack. One side may move up or down in relation to the other. The two sides may slide and tear against each other horizontally. The friction of the rocks grinding one side on the other often breaks up the rock into smaller pieces. Compared with the solid rock strata, the area of smaller fragments is very water permeable.

Fault breccia compared with normal rock layers at Mewslade bay.

It is thought that the entrance to Mewslade Bay is a fault gully, which has probably been eroded out by glacial meltwater during a period of permafrost when an ice sheet covered most of the Gower Peninsula. The various rock types represented in the gully structure would have been derived from both the Carboniferous Limestone and the Old Red Devonian Sandstone geology fractured by the fault and carried down towards the sea by water. The name for this type of deposit is Fault Breccia when the pieces are large, and Fault Gouge when they are much smaller. Secondary minerals such as calcite can precipitate from groundwater circulating around the loose fragments, filling the empty spaces or voids and cementing rock together.

Fault breccia or fault gouge in a fault gully

Low down, nearest the ground, the fault breccia looks like tutti-frutti ice cream with rock fragments coloured pink, red, brown, yellow, and grey embedded in a crystalline calcite. Higher up, the fragments are consolidated – but without the all-encompassing white matrix.

Gallery of fault breccia rocks in the fault gully at Mewslade Bay

Calcite occurs in different forms. There are neatly formed crystal clusters as veins or in pockets. Most are basically white but some are stained orange with iron. Superficially, the crystals are often coated with green or brown algal bio-films. In some places, especially in tunnels and low down in the deposit, where the the rocks are washed by the sea and abraded by debris at high tide, the calcite is worn down to a smooth surface with just an outline pattern of the crystals and cleavage.

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Slide show of crystal calcite from the fault breccia at Mewslade Bay

View of fault gully from Mewslade beach.

COPYRIGHT JESSICA WINDER 2013

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