Field book of common rocks and minerals

by Frederic Brewster Loomis

Pebbles

When picked up either from brook beds, sea beaches, or the open plain, there are few forms of rock which tell a story of the past more completely than do pebbles; and any one, who enjoys reading a story written in form, structure and composition, will find in pebbles one of the most satisfying and at the same time testing exercises. The story may be complex or simple according to what has happened to the parent rock, and to that is added what happened since the pebble left the ledge where it was a part of a great mass. One must not forget to take into consideration where the pebble was found and the character of its associates. This sort of exercise is recommended to all interested in rocks. It will yield something upon first trying, and more on prolonged study; and the fullness with which it is done will test one’s knowledge of the meaning of rocks as nothing else will do. As a sample of this sort of exercise let us take the two pebbles illustrated on [Plate 71].

The upper one is a common quartz pebble picked up in a New England brook bed. Such pebbles are common all over the country formerly covered by the glacial ice sheet. It is crystalline quartz, but the individual crystals are not distinguishable, and such quartz is typical as the filling of veins. It therefore goes back to a time when the rocks were fissured, probably in connection with the folding accompanying mountain making far to the north in Canada. Into the fissures thus formed seeped the water which had been leaching through the adjacent rocks, and it was saturated with silica which it had dissolved from those rocks. In the open fissure the quartz was deposited as crystals, which grew finally filling the fissure and crowding each other so that all the faces were obliterated. The quartz vein was complete, but it must have been far below the surface of the ground. Time must have passed, thousands of years of it, until, in the weathering away of the mountain system, the many feet of overlying rock were removed and this vein was brought to the surface. As the quartz is harder than the adjacent rocks, the vein soon projected as a ledge. The effect of changes of temperature in alternately expanding and contracting the rocks developed cracks, into which water worked its way, and then the breaking was hastened by the expansion which takes place when water freezes, and in exposed regions is so effective, because the freezing and thawing are so often repeated. Finally an angular fragment of quartz was dislodged and lay on the surface, resistant to the solvent power of the rain. In this case this happened just before the advance of the great ice sheet. When that came to the place where the fragment lay, it was picked up along with all other loose material and partly shoved in front of, but probably mostly carried frozen in the ice, and journeyed one, two, three hundred, perhaps a thousand miles. This took many years for the ice moved only a few feet a day. Finally however it came to the point where the ice melted as fast as it advanced, and our quartz fragment was dropped at the front of the ice sheet along with other great masses of till. Here there was abundant water, partly from the melting of the ice, and partly from the storms which must develop where there are such contrasts in temperature, as there would be over the ice, on one hand, and over the bare land in front of the ice on the other hand. A torrent picked up our fragment and started it on a second journey, banging against other stones as it rolled along down the stream bed, every time it struck another stone bruising the corners which soon became rounded. Thus from time to time during high water the quartz fragment, becoming rounder every time it moved, journeyed down stream, until it came to the point where the stream emptied into a lake. Here the current was checked and the stone dropped to the bottom along with other larger stones to make the delta at the mouth of the stream. There it lay as long as the lake existed, and would be lying now, but that in New England a tilting movement of the land tipped the north end of the lake up and the water all ran out. Then the stream began to flow over its own delta and in time of freshet tore a channel down through the old delta carrying the pebble still further down, until it came to the level stretch which represented the old lake’s bottom and there it dropped the pebble in its bed. And there it was found and picked up to become the pebble which told the above story of its life, and to repeat it as often as anyone will look at it with a seeing eye.

The second pebble is quite a different one. It was picked up in a gravel bank along a railroad cut, just at the foot of Mt. Toby in Massachussetts, and the writer has used it many times to test his students, to see if they could read the story which it tells.

It consists of two sorts of rock, the one, angular fragments of a hornblende schist, the other, a fine-grained granite filling all the spaces between the fragments of schist, even in cracks less than a quarter of an inch wide. The schist is the older rock and in its first appearance represents a deposit of mud (clay and sand) on the floor of the ocean, well out from the shore, and somewhere off to the east of Mt. Toby, perhaps ten miles, perhaps more, from the place where it was found. This was back in early Palæozoic times, millions of years ago.

This deposit was buried by further layers of sediment on the sea bottom and cemented into a shale. Then during a mountain making period the region was folded, and the sediments were altered by the combined pressure and heat, our layer of rock becoming a hornblende schist. After that happened considerable time must have passed, but just how much is not indicated by the pebble, before another period of disturbance took place, during which this deep seated schist was faulted, and shattered to fragments along the line of breaking. This accounts for the angular fragments. Then into the fissure thus formed was pressed a molten magma, which while liquid enough to flow and be squeezed into every opening could not have been very hot; for not even the corners of the schist fragments are melted or altered, so as to appear any different from the mass of the schist. The molten magma cooled rather slowly, making a fine-grained granite. This must all have taken place far below the surface, or the magma would have cooled into a felsite or dense lava.

Again a long time must have elapsed, while the rock overlying our piece was eroded away, so it could come to the surface. Just about the time it did come to the surface, the Connecticut Valley was formed by a great block, 95 miles long by fifteen to twenty miles wide, dropping down six or eight thousand feet (probably not all at once but by one or two hundred feet at a time) between two north and south faults. This took place in the Triassic Period. Of course the streams then began to wash sand and stones of all sizes into the hole. Our pebble was one of these. While still an angular fragment, lying perhaps ten miles east of the Connecticut Valley, a stream started it moving, and as it rolled along the brook bed, it was battered and rounded to its present shape, and finally tumbled over a waterfall to the bottom of the great hole, which had been formed as described above. Here with other stones it formed part of a coarse gravel, coarsest near the sides of the hole, and finer toward the middle; for the material was further distributed in the bottom of the valley. Our stone stayed pretty near the side and was soon buried beneath hundreds of feet of similar material. The leaching water dissolved enough iron rust so that this acted on the lower layers as a cement and bound the whole mass into a conglomerate.

Here for some millions of years our pebble rested, while above it was piled sand and gravel and a couple of sheets of lava, until the hole was filled, and our pebble was near the bottom of the mass. Later movements of the land raised the whole region, fully six thousand feet, and erosion went on for other millions of years. The conglomerate and sandstone wore away faster than the metamorphosed rocks on either side of the filled valley, so that a new valley, the present Connecticut Valley, came into existence.

When our pebble finally came near to the surface on the side of Mt. Toby (a mound of conglomerate which somehow was protected and wore down a little less rapidly than the conglomerate on either side of it), it was just about the time of the glacial period. The great ice sheet went over the mountain removing all the loose material and some more of the solid conglomerate. This brought our pebble to the surface, but too late to be moved by the ice. However as soon as the ice left the Mt. Toby region, the rains fell, and in the further weathering of the conglomerate, the cement holding our pebble in place was dissolved and it was freed. At once a tiny brook started it rolling down the side of the mountain, a brook so small that when the pebble reached the foot of the slope it did not have power to carry it further. Here there gathered a fan-shaped mound of such pebbles, known as an alluvial fan. It rested here not over a couple of thousand years, when the Central Vermont R. R. cut a groove through the fan, using the material for ballast, and here the pebble was found and brought home.