The Glacial Sea Bed at Finchley and Muswell Hill:
Unshaded parts: London Clay, with Pebble-beds at Mill Hill and Totteridge. Valley gravels (chiefly re-assorted Glacial gravel).
This small tract of glacial ground consists, as the map shading will show, of two kinds of material. The one is gravel, the other clay. The gravel is the lower formation of the two. Each of these accumulations varies in thickness. In the Finchley railway cutting, the united thickness is about 35 feet.
These accumulations of gravel and clay contain the evidence of their origin and history. By a study of their composition and contents, and by a knowledge of the physical conditions Is under which similar accumulations are taking place in glacial countries at the present day, we are, able to explain Their presence in our district. We call date back the time and circumstances of their arrival here to a particular period of our country's physical history, and explain the mode of their transport hither from their distant parent rocks. We shall find that they were left at the spot where we find them to day during the great glacial submergence of the country, and that they served to form the sea floor of tile submarine Middlesex of that period.
The Glacial Gravel
The sandy beds of small iron stained fragments of rock which bear the name of gravel are familiar to the eye of the Londoner. But the name gravel is often indiscriminately applied to beds of rock debris of various character and history.The glacial gravel is to be distinguished from material which is sometimes confounded with it. There are both gravels proper and pebble gravels, and these are found lying in juxtaposition in the Finchley district.
Gravel consists chiefly of more or less angular fragments of rock. Pebbles, on the contrary, are fragments of rock which have become rounded. Again, the pebble gravels around London are mostly composed of flint, and are native, as it it were, to the locality. The gravels, on the other hand, although pebbles are found in them, are a very heterogeneous collection of angular rock fragments derived to a great extent from distant areas.
The glacial gravel at Finchley consists of sub-angular iron-stained chalk flints and blackish flint pebbles, all of which may come from not distant areas. Then come next in proportion the rocks from a greater distance. Among them are quartzite pebbles of various colours grey, pink, and whitish as well as pebbles of white quartz.
Fossil shells are but seldom found in the Finchley gravel. But near Hertford, about eight miles distant, the same gravel (which, extending over much of the Eastern and East Central Counties has been called the “Middle Glacial,") abounds with the species figured on page 16 (Nos. 4 and 5).
The fact that the gravel lies beneath the glacial clay, and is thus the lower of the two floors of the glacial sea bed in our district, shows that, in point of time, it was the earlier of those two successive accumulations of the glacial sea.
The gravel marks a distinct single stage in the earlier part of the great glacial submergence. While the materials forming the clay which overlies it have been accumulated in an unstratisfied form, those which make up the gravel have been spread out in the stratified form by the action of moving water.
|“From the fact that pebbles from the adjacent pebble beds (which originated anterior to the Glacial Period) make up a considerable proportion of the gravel material, it would seem that the submergence could not at this time have exceeded 400 feet." S. Y. Wood, Junior.|
The Glacial Clay
This is by far the more striking and important of the two glacial accumulations of our district. Judging from similar accumulations which are taking place in glacial countries at the present day, the glacial or boulder clay is the product of those potent meteoric agencies which now prevail within the Arctic and Antarctic circles.
The change of sediment in the glacial sea bed from gravel to boulder clay denotes that a different agency from that of moving water was now in operation to form the sea bed in this particular area. This change is accompanied both by a difference in the character of the debris which makes up the sea bed, and by the greater bulk of the transported fragments.
The spots where the glacial or boulder clay is always exposed to view are marked on the map, viz.:-
It should be remembered, too, that temporary excavations in all parts of the district are always to be seen. Sometimes these afford valuable exposures of local modifications of the Glacial Clay. Let us now suppose ourselves to be standing in the presence of one of the above named sections.
The clay before us is remarkably full of small fragments of chalk. These fragments were in the first place mechanically detached from the parent rock by glacier action. They were then carried down with the clayey sediment to form the glacial sea bed, in which, as we see, they lie undissolved to this day.
The amount of chalk which is thus mixed with the glacial clay is such as to give the whole accumulation a prevailing chalky character. This character serves to distinguish it uninistakeeably from all other clays. In fact, the distinctive name of this deposit is The Great Chalky Clay, a name which we shall henceforth use for it. As we trace it in the various excavations of our district, it will serve to tell us how great must have been the waste which the chalk hills of England suffered during the Glacial Period by the abrading action of glaciers and the general potency of the meteoric agencies.
Perhaps the spots where the chalk most preponderates in the glacial clay of our district are Tatterdown Lane (now called Tetherdown), Muswell Hill, Islington and St. Pancras Cemetery; and the Marylebone Cemetery.
The glacial clay itself the sediment which forms, as it were, the matrix in which the chalk and other coarse materials are embedded is brown in colour and more or less sandy. There is, however, an important and interesting exception to this character in the railway cutting from East End to Finchley. In this case there is a lower bed of dark blue clay.
Besides the smaller fragments of chalk, the glacial clay abounds with large fragments of rock. Here, at Finchley, are massive angular blocks, such as a man can hardly lift. Here, too, are the rounded boulders which give to the deposit the name of the boulder clay. In these Finchley railway cuttings, brick fields, and cemeteries, we find boulders of granite, and masses of sandstone, coal, slate, clay, limestone, and other rocks, mostly from the northern parts of England. Mixed with these, in heterogeneous confusion, are the fossil shells and other organic remains of as wide an area.
The shells which are most common in our district.
Almost every excavation in the clay yields these specimens of gryphea and belemnite. In the various cemeteries they may often be seen lying unheeded in the footpaths. At the lodge entrance of the Marylebone Cemetery, a numerous and interesting collection of fossils ornaments the rock-work. In the railway cuttings and brick-earth pits of the district, fossils are easily obtained. All of these, it must be remembered, are second-hand fossils, and not the fossilised remains of animals of the Glacial Period.
We have now examined the composition and contents of this remnant of the glacial sea-bed at Finchley and Muswell Hill. Here is the conclusion which is arrived at by those who are familiar with similar accumulations :-
1. This strange medley of rocks and fossils, brought from various and distant parts of England, is such as only glaciers and ice-sheets are known to accumulate in the course of their passage over the face of a country. We see before us at Fnchley the material of the great terminal moraine (moraine porofonde) of the ice-sheet of the` Glacial Period, left behind at a certain stage of the northward recession of the ice before the advancing glacial sea.
11. What we see at Finchley is not an upland valley moraine, such as the tourist sees in the form of huge mounds in the cwms of Snowdon and other mountainous districts. Instead of such an inland valley moraine, lying intact, as the vanishing glacier left it, we see before us the materials of a moraine which has been extruded beneath the sea from the land-ice, and distributed by means other than that of moving water over the sea-bottom.
Here and there it would seem that the original morainic form of these glacial accumulations at Finchley is not altogether lost. The materials still seem to he lying at the angle at which they might have been pitched down from the land-Ice. But a larger view of the facts leads to the conclusion that the glacial clay, where we find it unstratisfied, has been formed entirely from the droppings of ice-bergs, which, as they became detached from the glacier, carried away at their base large freights of the moraine profonde.
A list of the fossil shells and other organic remains found in the glacial beds at Muswell Hill and Finchley, has been drawn up by Mr. Wetherell, of Highgate, the earliest glacial explorer of the district, whose collection is the result of more than thirty years of investigation. Geologists will be interested to know that in this collection the rocks most represented are the Chalk and the Oxford Clay. The rocks least represented are the Gault and the London Clay. Mr. Wetherell's interesting collection of Drift fossils is now in the Museum of Practical Geology at Jermyn Street. (Author’s footnote). ( Where they are now I do not know. J, Whitehead).