Halls Harbour - Rivers of Stone EarthCache

Halls Harbour - Rivers of Stone

Hall's Harbour is a fishing community in the Canadian province of Nova Scotia, located in Kings County on the North Mountain along the shore of the Bay of Fundy. Hall's Harbour is believed to be named after Samuel Hall, an American privateer in the American Revolution who used the cove to raid settlements in the Annapolis Valley but was forced to flee and abandon his ship in the Harbour by pursuing militia in 1779.

Visiting Halls Harbour you can explore the Triassic rock for formation along the shore line. Now this scenery is beautiful but when formed was a hothouse climate gripped in Pangaea leading to widespread desert conditions. This site lay in a parched rift valley far from the ocean.

At the end of the Triassic period, 201.3 (+/- 0.2) million years ago, tectonic forces pulling the Pangaea apart created a vast network of cracks from which lava emerged and spread out over parts of what is now South America, Africa, Europe, and North America which includes Nova Scotia. Remnants of the aptly named flood basalt can be seen along the Fundy south shore for more than 200 kilometers, from Cape Split to Briar Island.

Unlike the explosive volcanic rocks found in some other parts of Nova Scotia the rock here formed from red hot molten lava that flowed from cracks in the ground. Similar eruptions take place in present day Iceland.

The Jurassic North Mountain Basalts, Nova Scotia

More Than Just Simple Flood Basalts the 201 Ma, North Mountain Basalt (NMB) is a sequence of continental tholeiitic (common) basalts erupted within a continental rift(Fundy Basin) that has been subdivided into lower, middle and upper flow units (LFU, MFU, UFU, respectively).

Lower: The LFU is a medium- to coarse-grained, massive, columnar jointed basalt of dominantly holocrystalline texture with minor vitrophyre. Glassy look. This was deposited first.

Medium: The MFU contains numerous variably thick flows with a laterally variable composite thickness (150 to 10 m) that decreases westwards. The basalts are fine- to medium-grained and contain abundant, variably textured . The MFU is characterized by a systematic zonation of zeolites occluding primary vugs (cavity).

Upper: The UFU consists of at least two or more flows and is similar to the LFU. It is a massive, columnar jointed basalt with a medium- to coarse-grained texture and contains abundant vitrophyre (rock with different size crystals). Segregation pipes of 3-5 cm to 1 m occur locally and a pervasive early silica alteration along columnar joints post-dates pipe formation. The distribution of the UFU is poorly constrained with the greatest exposure at the west end and the bottom part only observed along the length of the valley coast line; it is not exposed along the north side of the Fundy Basin.

The age of a rock can be determined using two methods: relative dating and radiometric dating.

Relative Dating

One of the most common questions is how do we know the age of the rocks? This form of dating is very intuitive. It does not give precise dates but rather the age of rocks relative to other rocks. Here are some of the principles of relative dating using pancakes as an example.

1. Principle of Superposition: Rocks were originally layered with the oldest on bottom. Imagine you are making pancakes. When you stack the pancakes, the first pancake to be made will be the bottom layer. Each pancake layer going up is fresher than the last.

2. Principle of Original Horizontality: Almost all rocks were deposited in a horizontal layer. The pancake batter spreads out in a horizontal layer on the pan.

3. Principle of Lateral Continuity: Layers of rock are continuous until they eventually taper out. Pancakes are a continuous layer that tapers out at its edges.

4. Principle of Inclusions: Any rock fragments found inside another rock (inclusion) is older than the host rock. Blueberries inside a pancake had to have been grown before the pancake batter was made.

5. Principle of Cross-Cutting Relationships: Folding, faulting and other deformation events are younger than the rocks. You can only cut into pancakes after they have been made.

6. Principle of Fossil Succession: Rocks that contain the same assemblage of fossils are of the same age. Fossils are the remains of biological organisms that evolve and change form over time. The forms of fossils within rock can help to identify its age. Unfortunately, this is not easily explained with pancakes.

Radiometric dating

Radiometric dating is a method of establishing how old something is perhaps a wooden artefact, a rock, or a fossil based on the presence of a radioactive isotope within it. The basic logic behind radiometric dating is that if you compare the presence of a radioactive isotope within a sample to its known abundance on Earth, and its known half-life (its rate of decay), you can calculate the age of the sample.

Flood basalts

Lava covered the region in three major episodes totaling as much as 500 meters in thickness. At Halls Harbour you can view multiple lava flows from the second episode. In the cliffs there are signs that the core of one flow once bubbled and frothed with volcanic gases, and at the bottom of the overlaying flow chilled as it advanced across the ground.

The chilled, uniform base of lava rests on the weathered top surface of an older flow in the cliffs along the shore. If you walk along the cliff line for about 100 meters you can see the two different lava layers.

The upper one spread on top of the older one making the top the younger layer. Before the upper layer came, the bottom layer got weathered and stained. It became weakened by the harsh desert environment. Along the contact between the lava flows, you can see the weathered rock below and the solid looking base of the younger flow below. The younger lava cooled quickly against the ground and for this reason it is very fine grained.

Parts of the lower flow have a dotted appearance because it bubbled with volcanic gases that left voids as the lava cooled. Later, circulating fluids deposited quartz and a variety of zeolite minerals. Including Stilbite in the voids, creating the colorful amygdaloidal texture. Some zeolites have reacted with sea water, acquiring a bright blue-green coating.

To log this Earthcache visit the viewing location. Please answer the following questions and send in a timely manner to my geocaching profile or email. Answers not received will result in deleted logs.

Needs to be completed at half to low tide to access the lava cliffs safely

1 - What is the height of the rock face?

2 - At what height does the weathered layer start?

3 - Is the top layer the same thickness all along the rock face?

4 - Which layer is older and why?

5 - Describe the bottom layer at ground zero in terms of colour, texture, minerals, and coarseness.

6 - What colors of stones did you find in the layer at ground zero and why the different colours?

7 - Why does the bottom layer have so much weathering?

8 -Post a picture in your log with a personal item or hand in picture to prove you were there.

[REQUIRED] In accordance with the updated guidelines from Geocaching Headquarters published in June 2019, photos are now an acceptable logging requirement and WILL BE REQUIRED TO LOG THIS CACHE. Please provide a photo of yourself or a personal item in the picture to prove you visited the site.