GEOLOGICAL TIME

Introduction

People have always been interested in time and its passage, looking ahead to future things and back to what has already taken place. In a very early chapter we showed that physicists, astronomers and geologists have come to the conclusion, from different lines of evidence, that the earth is between four and five billion years old. Thus, in geology, we have to deal with a very long period of time compared to the three score years and ten of a human lifetime, and we realize from this what a tiny fraction of earth history that each one of us will see.

In human affairs we divide time in two separate ways—the absolute way and the relative way. The absolute way depends on an accurate measure of time. Thus, we may say of an event that it will take place in two hours and twenty minutes or that it took place 241 years ago. The relative scheme uses major happenings, so that we date an event as being pre-war or post-war, as being pre-Confederation or post-Confederation. Until recently, when a small beginning has been made, no accurate and widely applicable system of measuring absolute time has been available to the geologist. On the other hand a record of the earth's past is preserved in the rocks and has shown many events of world-wide importance. It was, therefore, natural that geologists should have developed a relative calendar for the measurement of geological time.

Great Events of the Past

What are these events and what kind of record could they leave in the rocks to tell us of their happening? The earth's history seems to be punctuated at irregular intervals by great upheavals of parts of its framework. This kind of thing the geologist refers to as mountain-building because mountain systems result from the folding and buckling of the earth's outer layers. No one really knows why these great periods of disturbance take place. They may be related to the shrinkage of the earth as it cools down. They may be related to movements of the continents on some sort of a mobile or plastic substrate. Regardless of why they take place, they do take place, and these great events make convenient dividing lines in time.

The earth's history, too, is periodically marked by great volcanic outpourings. These seem to be related in some places to the periods of mountain-building we just mentioned. These outpourings leave behind them great masses of volcanic rocks, ashes, and dust, which the geologist can recognize even millions of years later.

Even more important are events related to the seas. You may recall how the ocean basins are quite well marked from the continental masses which rise from the ocean depths. The edges of the continents are gently sloping for the most part, and, coupled with the fact that the ocean basins are full to the brim, this means that the slightest adjustment of levels of either one will alter the distribution of shallow seas, which sit on the continental mass even as Hudson Bay and the Baltic Sea do now. The geological story is filled with chapters which seem to begin with the spread of shallow seas on the continents and end as they withdraw again.

The Fossil Calendar

Still another series of events which provides a method of dating rocks on a comparative calendar relates to living things, and their remains which are found in the rocks. Creatures which live in certain environments may leave behind them hard parts, like bones or shells, which will serve to identify them exactly. Thus a clam may live on a muddy bottom in the sea. On its death, the soft fleshy parts may disappear but the hard shell remains. As muds accumulate on the bottom of the sea the shell may become deeply buried. The muds turn to rock with the passage of time and still the shell remains identifiable. Such remains of ancient living things in rocks are called fossils. They provide us with a cross-section or sampling of the life of the time at which the containing rock was accumulating as a sediment. If this were the whole story we would be no farther ahead in dividing geological time. We have found, however, in 170 years of careful study that the oldest rocks are without traces of living things, that recent rocks contain fossils of things much like those of today, and that rocks in between are full of remains of organisms which show that life was different from what it is now. As a result we believe that life has changed since it first appeared on the surface of the earth, through a traceable sequence to its present-day state. This means that we have a changing pattern of life across a period of time; fossils which show what the pattern was at the time the containing rock was laid down as a sediment; and, therefore, a way of fixing the age of that rock relative to the changing pattern.

The very beautiful snow and glacier covered northern slopes of Mount Robson with Berg Lake below supply this spectacular view from the western boundary of Jasper National Park. The horizontally layered rocks of Precambrian and Cambrian age are typical of most of the famous peaks in the Canadian Rockies.

In review, then, we have several kinds of events by means of which it is possible to draw up a time-table to mark the geological past; mountain-building episodes in which great folding, faulting, and upheavals took place; volcanic outbursts in which massive outpourings of lavas, ash, and dust mark the event; invasions of the continents by shallow marine waters which leave their record in the rocks; and a changing pattern of living things which means that rocks which contain their remains may be dated relatively by them.

The Latest is on Top

Even if none of these methods were applicable we could still draw up a sort of geological calendar based on the Law of Superposition, a grandiose title for a simple principle. Very soon after it was realized that sedimentary rocks are formed from muds, sands and gravels on the bottom of the sea or in river deltas, it was realized that the last ones to be formed lie on top of those which were formed earlier. This seems very sensible, for if we scatter layers of sand on the floor, one after another, the last one should be on top of the pile and the first on the bottom. This is all there is to the Law of Superposition—in a sequence of layered rocks the youngest lie on top and the oldest on the bottom, so that more recent geological events would be recorded in the younger or top rocks and older ones in the ones underneath. Now when you look into the Niagara gorge or any other river valley, or at the great walls of rock in the western mountains, you may recognize that the rocks in the upper parts were laid down after the ones in the lower regions, perhaps thousands or even millions of years later. Even if mighty upheavals of the earth's outer layers disturb the flat-lying rocks and tilt them out of their original positions the Law of Superposition still holds in a broad way. In a few, rare localities, sequences of layered rocks are actually turned upside down, but in these places folding and faulting are so severe that the geologist is on the lookout for the exception to the general law and uses other methods.

From all these sources of evidence, then, a history of the geological past was built up from the record without benefit of any way of telling absolute time, but only with ways of telling the order in which ancient events took place, and something of their size and importance.

Radioactive Clocks

The measurement of geological time on an absolute scale is a complicated physical and chemical exercise in techniques but like many complicated technical processes it is based on a fairly simple principle. Certain minerals are subject to radioactive decay as soon as they are formed. Radioactive decay takes place at a constant rate and produces recognizable by-products. Thus, if the ratio of the by-product to what is left of the original mineral material is measurable, and if the rate of decay is known, then an accurate measure of the time which has elapsed since the mineral was formed is possible. Several different minerals with several different radioactive elements in them provide us with several different methods of measuring geological time on the absolute scale. Measurements of time in this way have confirmed to a surprising degree the relative calendar.

The Geological Calendar

It is time, now, to have a look at the geological calendar we have been describing. Like any other calendar it has time units of different sizes on it. It is also filled with strange names but this is understandable in view of the way it was developed over a period of time by the relative methods we have described and by different workers in different parts of the world. Thus the Devonian period is named for Devon, where the rocks which represent that period of time are reasonably well exposed and where they were first described. Others have been named for other places for similar reasons and include Permian for the Russian province of Perm, Jurassic for the Jura Mountains. Some of the names of the periods come from the kind of rocks commonly found in them in their original localities. Thus we have Cretaceous, from the Latin name for the chalk which characterizes rocks of that age in Britain and France, and Carboniferous for the period in which coal was commonly formed.

In the accompanying geological calendar you will find the names of the time units, how long each was and how long ago, and something of the events which took place in those periods. It is important to think, as you read it, of the way in which it has been drawn up and that the figures on it are pretty accurate but really are intended to give us a general idea of when these ancient events took place.

Living Things and Their History

From the geological calendar we can see that the history of life on earth extends back into the shades of the past perhaps a billion years. Fossils show us that a great change seems to have taken place at about the beginning of the Cambrian, when fossils suddenly appear in great numbers and in greater variety than in earlier rocks.

Now the living things which are found in abundance as fossils in the Cambrian are very different from the creatures we find about us now; different in two ways—different in kinds and different entirely in the balance of numbers. Nowadays we think of the vertebrates, the animals with backbones like fish, reptiles and mammals, as being the dominant and most successful creatures of the time. In the Cambrian, vertebrates were as yet unknown. Further, the lands stood treeless and naked and the seas were apparently dominated by a couple of forms of invertebrate life, the one, brachiopods, somewhat like clams in their organization, and the other, the trilobites, a group of long-extinct crustaceans, or lobster-like creatures.

The story of the gradual change from these modest beginnings in the Cambrian, some 500 million years ago, to the diversity and complexity of modern life is one of the most fascinating of all branches of knowledge. The first forests spread over the land late in the Silurian period, perhaps 340 million years ago, and coincided more or less with the rise of the fishes in the oceans of the day. Great forests of trees, now extinct, spread over enormous swamps marginal to the seas of the Carboniferous period some 270 million years ago and gave rise to vast deposits of coal in many parts of the world.

The Triassic and Jurassic, beginning 200 million years ago, were marked by the amazing success of the reptiles which culminated, in the Cretaceous period, in creatures of enormous size and variety, including the dinosaurs. These creatures have been extinct now for 60 to 70 million years. They were replaced at the end of the Cretaceous by the mammals which have gradually developed to their present-day variety and specialization of purpose and habitat. The appearance of the flowering plants dates only from the same Cretaceous period. And our own ancestors? Well, the best estimates place the first primitive man on the very top leaf of our geological calendar, probably within the last two million years.

The Geological Calendar. (click on image for a PDF version)

Scenery on the Calendar

The modern scene around us results from a compounding of very ancient events with very recent developments on the geological calendar. The lakes we go to in summer may have been produced by the glaciers disrupting the drainage of the area within the last 50,000 years, but the rocks which enclose them may be a billion years old. The Falls of Niagara may have been made only 15,000 years ago yet the rocks which make them possible are of Silurian age and were laid down in the seas of 350 million years ago. The rivers which roar through the canyons and steep valleys of the western mountains may be but a few tens of thousands of years old, yet their valleys are cradled in rocks which range from the billion-year-old Precambrian to the nearly recent. The whole complex of living things on the earth at present is the culmination of a gradual changing pattern or evolution from beginnings lost in the haze of time but which must have been nearly a billion years ago. The whole subject of geological time and the enormous span of years which we have to deal with makes man's appearance on earth a very recent event indeed.