At the posted coordinates, you will examine a road cut of what appears to be green marble. There is safe parking at N 45° 09.443, W 76° 31.072’ and the road cut is far enough away from the road to be relatively safe. The cut is located at the bottom of a hill at the end of a curve. Traffic is light but there are large trucks passing by on occasion. Since is this a road cut on a public road (Crown Land), permission is not required to go there.
A VERY BRIEF GEOLOGICAL HISTORY OF ONTARIO
The geologic history of Ontario dates back 3,250 million years ago. The province is divided into three geological provinces: Superior, Southern, and Grenville. The three geological provinces form part of the Canadian Shield. Eastern Ontario is part of the Grenville and the Paleozoic-Mesozoic Basins, the St. Lawrence Lowland. The rocks we see in Ontario represent ancient ocean floor, deep crustal sections, remnants of ancient mountain ranges, deep sea sediments and ancient and modern glacial deposits. The Superior province was the first to develop and it covers the majority of Northern Ontario. The Southern Province was accreted to the Superior Province and evolved from 2,490 to 570 million years ago.
The Grenville is the geological province of interest to us for this EarthCache. It is the third stage in the formation of Ontario, built from approximately 1,760 to 1,000 million years ago. This geological province was the site of a series of mountain-building events, the Grenville Orogeny (Oro is Greek for mountain; geny is for genesis), which occurred between 1,180 and 1,000 million years ago. The rocks of the Grenville were baked, twisted, squeezed, and stretched by the orogenic and volcanic events in the region. The Grenville occupies the “mid-section” of Ontario, from the St. Lawrence to just south of Sudbury. It is composed of two main areas; the Central Gneiss Belt and the Metasedimentary Belt, each comprised of different terranes. A shallow tropical sea covered the area approximately 1,240 million years ago.
Regional Geology of Lanark County
The area that includes Lanark Township is underlain by Precambrian rocks of the Central Metasedimentary Belt (CMB) of the Grenville Province. The oldest rocks were deposited starting 1300 million years ago followed by clastic (pieces) and carbonate sedimentation and volcanic/ sedimentary cycles, which repeated over time. A variety of plutonic rocks appeared between 1,250 and 1,100 million years ago and younger sediments were deposited about 1,050 million years ago.
The Grenville Orogeny culminated about 1,000 million years ago. Carbonate sedimentary rocks are wildly distributed throughout the CMB in southeastern Ontario forming marble belts several kilometres wide and tens of kilometres long. Layering is a common feature of the Grenville marble reflecting compositional differences, calcitic or dolomitic.
Colour of the marble is related to the impurities in it. Most carbonate sediments contain at least a small amount of detrital quartz and feldspar which reacts with the carbonates during metamorphism to form silicates. The minerals are often concentrated in layers giving a banded appearance. Sulphides, such as pyrite, are also locally present. Some marble contain impurities such as amphibolite which are in bands but are more commonly boudinaged (rounded shape; from the French word “boudin”).
Lanark County is a somewhat unusual place, geologically speaking, as it has large deposits of marble. The Grenville Province in south-eastern Ontario contains bands of high-purity calcitic and dolomitic marbles.
MARBLE OUTCROP, TATLOCK, ONTARIO
This outcrop is somewhat eye-catching as it appears pale-green. Close examination reveals that the outcrop of marble contains green and grey veins of some form of minerals. The marble is the result of the metamorphism of carbonate rocks such as limestone. Some other minerals have also formed in there, mostly the result of further transformation, this time due to hydrothermal fluids that escaped from the metamorphosed rocks. This further transformation is called metasomatism. With the marble you will find other minerals: some dark green, some pale green, some grey.
IDENTIFYING MINERALS
To identify minerals, we need to do some basic tests, using basic materials. You will need a hand loupe with a high magnification (at least 5). When I started, I found that a stamp loupe (x10) works fairly well and is not expensive. To use the stamp loupe, reverse it and put the base around your eye. Then get close to the rock or bring your sample close to the loupe until it is in focus. You will also need some vinegar and a steel nail.
Here’s a list of potential minerals that can be found at the site:
Hornblende (amphibole group): Grey to black colour, somewhat vitreous lustre, cannot be scratched by a steel nail, breaks in flat planes that are 120° from one another.
Calcite: Marble is made of calcite (calcium carbonate) that is left over from the metamorphism of limestone. Colourless, white or different colours depending on the impurities present. It is soft and can be scratched with a steel nail. It will react a bit with vinegar but the powdered from will give a good effervescence. The gas released is CO2. Calcite tends to break in three flat planes not at 90° from each other.
Dolomite: This is calcium carbonate with magnesium present. Colourless, white or different colours depending on the impurities present. A bit harder than calcite, it can still be scratched with a steel nail. It will not react with vinegar.
Serpentine: Typically olive-green or dark green, looks waxy, has a greasy feel, somewhat soft, can be scratched with a steel nail, will not react with vinegar.
Graphite: Grey, shiny, very soft, can occur in flakes that may only be seen with a good loupe.
Talc: Can be white, or apple green. It looks and feels greasy, a bit like soap. It is very soft and it is very easily scratched with a steel nail. It does not react with vinegar but the calcite around it may react.
TO LOG THIS CACHE:
Go to the site and answer the question, below, to the best of your ability. To answer the questions, you will need to do the basic tests listed in the mineral descriptions. E-mail me the answers before you log the find. Please, do not include any spoilers.
1. Why do you think that this outcrop has a greenish tinge to it? What mineral is doing this? Do your tests to confirm that it is what you think and not something else.
2. As you explore the outcrop, you will find some very interesting, and I admit beautiful, patterns of grey and green. Look at the grey bands closely and do your tests. What do you see with your hand loupe: hornblende or graphite specs?
3. It is okay to take pictures of the interesting patterns that you find and post them with your log. Pictures will not spoil this cache. Here’s a hint: when you take a picture, have something of known size in it, like a coin or pen, to give an idea if scale.
Please do not break the surface of the outcrop to take samples. Nature has done this for you and there plenty of fragments on the ground for you to collect.