In the Paleozoic Era, during the Ordovician period, approximately 425 - 500 million years ago, a tropical sea called Lake Agassiz covered Manitoba. Limestone was formed under water by the action of organic or chemical agencies, or a combination of the two, on dissolved matter, primarily composed of calcium carbonate (calcite) and magnesium (dolomite) that was deposited in layers or beds separated by a layer of shaly material. Remains of corals, snails, cephalopods, trilobites, and brachiopods that lived in this early sea became part of the sedimentary layers.
Consolidation into solid rock may have been brought about by a number of factors. Chief among them is the growth of cement crystals of calcite, or dolomite, throughout the mass. It may also have been brought about by the weight of the material itself or of beds of other material deposited on top. Pressure due to earth movements played an important part of consolidating the deposits, and the heat of igneous intrusions was also effective.
Due to the earth’s movements, glaciation, and the receding of Lake Agassiz, the deposits were raised to dry land creating a limestone belt, about 100 miles wide, extending diagonally across the province. The limestones lie in even beds having a slight prevailing dip to the southwest at a very low angle. Manitoba’s limestones include practically all types from pure high calcium limestone through magnesian limestone to pure dolomite. It is used for a variety of purposes, including the manufacture of cement, lime, and building stone.
Tyndall Stone is defined as a medium density dolomitic limestone. Geologically, it is referred to as the Upper Mottled Limestone of the Red River Formation of the Ordovician System. Tyndall Stone is found on the Red River Plain, one of the flattest regions in all of North America, thirty kilometers northeast of Winnipeg, in Garson, Manitoba.
The ground mass or matrix of the stone is fine grained high calcium limestone, containing many fossil fragments composed of calcite. Weaving through the matrix is a high magnesium material consisting of dolomite in tiny rhombic crystals cemented with calcium carbonate. The dolomite is what gives Tyndall Stone its’ unique mottling or “tapestry” appearance. No satisfactory explanation can be given for the formation of the mottling, but go back to the Introduction and view the animation. It is one possibility.
Textures of limestones vary considerably. Some limestones are compact, others are porous and full of cavities, still other are composed of shells and fossil remains knit together with varying degrees of firmness. All limestones are crystalline, composed of crystals of calcite and dolomite. In the groundmass of calcium limestones the crystals are rarely present in their true symmetrical forms but usually shapeless grains tightly interlocked. Dolomite crystals, on the other hand, are usually well shaped. Common sizes of individual crystals range from microscopic dimensions up to ¼ inch in diameter. It is the size of the glistening facets of freshly broken crystals that gives limestone the appearance of being either fine or coarse in grain, which is how limestone is classified.
Tyndall Stone is classified as a fine grain limestone which means that the crystals are visible, but under 1/32 inch in diameter. The crystals also have light reflective qualities, which gives Tyndall Stone an approximate light reflectance of 70 – 82% depending on the finish.
The strength of a stone is usually expressed in terms of the resistance to crushing, tensile, shearing and transverse stresses. The average crushing strength of Canadian limestones compares well with that of any class of building stone. Tyndall Stone has a crushing strength that varies from 7,500 to 2 10,020 lbs per square inch and a transverse or flexural strength of 1,340 lbs per square inch.
The greatest strain liable to be thrown on a stone in a building is either a shearing or a bending stress caused by foundations settling in an unequal manner, or by poorly bedded joints. Tyndall Stone has a respectable shearing strength of 1,055 lbs per square inch. Tensile tests made on various stones in other countries show that as a group, limestones are superior in tensile strength to any other group of rocks except the slates.
Sound retardancy is excellent, and Tyndall Stone is also an incombustible material that calcines at temperatures above 1500 degrees Fahrenheit.
NOTE: The physical and chemical properties vary somewhat depending on whether the stone sample being tested is buff or grey, wet or dry, or contains more or less mottling. The preceding test results are representative averages.
No limestone deposit is entirely free from impurities. Impurities are not always undesirable and, in fact, limestone containing certain impurities is preferred for some purposes. No limestones widely used for cut stone have a large quantity of impurities present. A description of the principal minerals and impurities of limestone follows.
Calcite is the essential constituent of all limestone. It is usually present as irregular grains instead of definite rhombohedron crystals.
Dolomite is the double carbonate of calcium and magnesium, is named after the French geologist Dolomieu, who announced some of its characteristics in 1791. Like calcite it commonly occurs in rhombohedral crystals, but the faces of the dolomite crystals are often curved. It is invariably present to some extent in all limestones.
Silica is present in all limestones in proportions varying from a trace to very large quantities. It may occur in visible forms as sand grains, chert nodules, and silicified fossils.
Alumina is present in combination with silica in the form of shale or argillaceous matter. It may occur as mere traces disseminated throughout the stone, or in thin films along the bedding planes.
Iron occurs in limestones principally in the form of oxides and sulphides. In chemical analyses, however, it is commonly recorded only as the ferric oxide. Iron minerals are normally present only in small amounts but some of the oxides are pigments to which, in large measure, is due the color of the stone.
Sulphur occurs in limestones principally in combination with iron as either pyrite or marcasite. It is also present in the sulphate form combined with calcium and more rarely with magnesium.
Organic matter resulting from vegetable matter deposited with the limestone, and from the soft parts of the creatures whose shells and skeletons compose a great part of the rock, is a common constituent of limestone. Carbonaceous matter even in very small amounts distributed through the stone acts as a pigment and to it most of the black and dark grey limestones owe their color. The partings or thin films of shale between beds often contain a high percentage of organic matter.
Calcium Phosphate varies in quantities from a trace to rarely over 0.50 percent occurring in nearly all Canadian limestones. The form in which it occurs has not been determined, but it is possibly an original constituent of the shells that form the limestones.
Chemical analysis gives the following breakdown of Tyndall Stone’s components:
|Based on average sample of
buff or grey stone with average
density of mottling
The abundance of large and well-preserved fossils in Tyndall Stone is another interesting component of the stone. Because of this, Tyndall Stone is sometimes referred to as a fossiliferous limestone. The fossils are more prevalent in some localities than others and are also particularly characteristic of certain beds. Many of them are relatively inconspicuous and serve only to add interest and character to the stone. The most common fossils found are:
|Tabulate (Honeycomb) Coral
Halysite (Chain) Coral
Streptelasma (Horn) Coral
Receptaculite (Sunflower) Coral
Brachiopods and Pelecypods
Distribution and Character
Tyndall Stone has been quarried in the areas of Garson, Selkirk, Lower Fort Garry, and Tyndall. Its’production has come mostly from a band of limestone 22 to 26 feet thick near the middle of the Upper Mottled formation. Beneath this horizon, nodules of hard, blue chert become increasingly numerous and spoil the limestone for use as cut stone. The stone deposit lies in layers or beds that are usually separated by a thin film of bituminous shale or by calcareous shale called a parting plane. Each bed represents a period of uninterrupted settlement and each film of shale represents a break in the process. The top layer of the deposit is located from eight to fourteen feet below the surface of the ground at the present quarry location in Garson, Manitoba.
The top eight to twelve feet of the stone is much lighter in color than that below, the matrix being a
4 light creamy buff with pastel brown mottling. From fourteen to sixteen feet is a variegated mix of buff and grey matrix, and in the remaining layers the matrix is a pale grey with grey-brown mottling. Rarely, an anomaly of a golden buff meanders vertically through the layers of buff and grey. The buff color beds range in thickness from twelve to twenty-six inches and the grey color beds range from twelve to thirty-six inches. These measurements refer to the thickness of stone free from parting planes.
There is every indication that the change in color is due to the action of downward percolating groundwater on the organic matter and pyrite present in the stone. The change proceeds to unequal depths but usually stops at a bedding plane. There are some nodules of white chert present in the stone, particularly in the grey color beds. They occur in a distinct streak running parallel to the bedding plane.
Joints and Bedding Planes
The commercial value of any stone deposit depends largely on the character and spacing of the natural structural planes that divide it into blocks of various shapes and sizes. In undisturbed strata, such as building stones, the most important of these are joints and bedding planes.
A joint is any one of a series of cracks that occur in all strata generally at a steep angle to, or vertical to the bedding planes. The striking characteristic of joints in limestone deposits is their frequent occurrence in parallel systems approximately at right angles to each other and the bedding planes, thus dividing the deposit into a series of nearly rectangular blocks. The spacing of joints varies from a few inches to many feet and upon this feature the value of the deposit and the method of quarrying greatly depends. Dip joints are usually a joint system that runs roughly parallel to the direction in which the strata are inclined. Those of the system at right angles to this are referred to as strike joints. Generally the strike joints are more prominently developed than the dip joints. Master jointscut through a number of strata and extend for a long distance and to considerable depth; whereas minor joints may be confined to a single stratum. When very imperfectly developed, certain of these minor joints are referred to as dries. Jointing is always more marked near the surface of the deposit.
A bedding plane is the line of demarcation between any two of the strata that make up a sedimentary deposit. A film of shaly or bituminous material almost invariably accompanies bedding planes in limestone. As a rule, the bedding planes in limestone deposits are nearly parallel, but in some cases are quite irregular, sufficiently so to make it difficult to secure rectangular blocks in the quarrying process. Bedding planes may be flat or even, they may be wavy, or they may be stylolitic. Usually the flat or wavy ones constitute a free parting along which the beds may be easily separated or pried apart.
Stylolites are particularly characteristic of limestone. The name is given to a zigzag-bedding plane that resembles the sutures or bone joints in a skull. Quarrymen call them “crowfeet”. The stylolite is really composed of little interpenetrating columns of limestone from adjoining beds. In height they vary from a fraction of an inch to several inches. Stylolite planes are usually parallel to the strata of the bed, and the columns are at right angles to the bedding. Some are only a few inches long, others, extend for long distances. Stylolitic bedding planes, owing to their interlocking character, do not form free partings and the beds have to be wedged apart – to use a quarry term, the beds are “strapped”.
Following is a sample of what an exposed bed layer of Tyndall Stone in the quarry may look like:
|Clay soil containing many fragments of limestone.
|Broken beds of buff limestone, 6 to 8 inches thick.
|Buff bed containing nodules of decomposed chert.
|Buff bed, the top 3 inches of which contains several minor partings.
|Buff bed with a stylolitic parting, 10 inches down from the top, which in places divides it into two beds.
|Variegated buff and grey, with partings 3 and 9 inches down from the top.
|Grey bed for the most part, but it is variegated in some places.
|Grey bed except for bottom 3 inches, which is buff.
Prominent bedding plane containing clay.
|Grey bed except top 4 ½”, which is buff.
|Grey bed with an indistinct parting plane 6 inches down from the top.