The Principles of Stratigraphical Geology

by J. E. Marr

[45] The lines 1, 2, 3 ... are incorrectly drawn in the figure. Line 1 should be drawn so as to separate a, b, c, d from a¹, b¹, c¹, d¹, line 2 to separate a¹, b¹, c¹, d¹ from a², b², c², d², and so with the others.

If elevation ceased and were succeeded by depression, the exact opposite would occur, and the pebble beds would be overlain by sandstones, these by muds, and lastly limestones would appear. It follows that during a marine phase occurring between two unconformities we should have a V-shaped accumulation of deposits with the apex pointing to the part of the shore line which was last submerged before the commencement of elevation, as shewn in [Fig. 14], though the beds of the apex will in most cases be denuded during the re-emergence.

Fig. 14.

Indications of the non-coincidence of the planes separating faunas and those which separate deposits of one lithological character from those of another have already been detected, for instance the 'greensand' condition of the Cretaceous period occurs in some places during the existence of one fauna, and in others during that of another, though the planes have not been traced continuously. Mr Lamplugh has furnished another example amongst the Cretaceous rocks of Yorkshire and Lincolnshire, but as has already been observed, a great deal remains to be done in this direction, and geologists are much in want of two sets of stratigraphical maps, in one of which the lines are drawn with reference to the differences of lithological character, whilst in the other they separate different faunas.

The student will notice the normal recurrence of deposits in definite order; conglomerate succeeded by sandstone, mud and limestone, in a sinking area, and limestone succeeded by mud, sandstone and conglomerate in a rising area. Naturally many instances of departure from this rule are seen, owing to local conditions, but on a large scale, it is very frequently noted, and recognition of this will enable the student to remember the variations in the lithological characters of the deposits more easily, than if he simply acquired them from a text-book without taking heed as to their significance.

Upon the variations in the lithological characters of deposits and of their faunas, when the beds are traced laterally depends very largely the successful ascertainment of the existence of former coast-lines, the restoration of which constitutes an important part, of Palæo-physiography, concerning which some observations may here be made[46]. If a set of deposits having different lithological characters can be proved to be contemporaneous, the coarser detrital accumulations will point to the approach to a coast-line, and the actual position of the coast during the period of accumulation of the deposits may be very accurately fixed. The pebble-beds at the base of the Cambrian rocks of Llanberis indicate the existence of a coast-line in that position during the accumulation of those pebble-beds. Similar pebble-beds occur at St David's, at the base of the Cambrian, but it is impossible in the case of these rapidly accumulated sediments to say that two deposited so far away from one another were actually contemporaneous, and therefore although we might draw a line through Llanberis and St David's to indicate the old coast-line of the period, it does not follow that the actual beach existed simultaneously at the positions indicated. The palæo-physiographer, however, attempts to restore the physical conditions of greater thicknesses of deposit; for instance, the distribution of land and sea during Lower Carboniferous times over the area now occupied by the British Isles is often taken to illustrate the methods of restoration of ancient features, and all admit that the lithological and palæontological characters of the rocks indicate a shallowing of the Carboniferous sea when passing northwards towards Scotland. For conveying an idea of the restorations to the student, it is almost imperative to portray the distribution of land and sea upon a map, and this can only be done by drawing definite lines. It must be distinctly understood that these lines are necessarily only an approximation to the actual position of the ancient shore-lines, which must have shifted again and again during the long period occupied by the accumulation of the Lower Carboniferous strata, so that a true idea of the positions of the Lower Carboniferous shore-lines could only be obtained by placing on a series of maps the successive shore-lines of different parts of the Lower Carboniferous period, and taking a composite photograph of these, which would appear as a wide belt of shaded portion of the map with no definite boundaries. The utmost that the maker of palæo-physiographical maps can expect to indicate, when dealing with considerable thicknesses of strata, is an approximation to the mean position of the shore-lines of the period when these strata were deposited. This is extremely valuable in enabling the student to understand the significance of the variations in the characters of the strata and their organic contents, if he distinctly recognises the generalised nature of the map. Examination of any two palæo-physiographical maps of the same period by different authors will shew wide divergences in the details, but a general resemblance of the main features. The reader will do well to consult Prof. Hull's restoration of the physical features of Old Red Sandstone and Lower Carboniferous Times on Plate VI. of his Contributions to the Physical History of the British Isles, and compare it with the map drawn by Prof. Green (Coal: its History and Uses, by Profs. Green, Miall, Thorpe, Rücker, and Marshall, Fig. 3, p. 38), which will be found to bear out this statement.

[46] On this subject, the student may consult Prof. E. Hull's Contributions to the Physical History of the British Isles.

Valuable as the published maps of palæo-physiography are as an aid to the student in understanding the significance of the variations of characters amongst the sediments, he will do well to supplement them by maps which he fills in for himself. He is recommended to procure a number of outline maps of England, or of the British Isles, and when studying in detail the characters of the British sedimentary rocks formed during the various periods, to place a blank map by his side when beginning the study of each period or important portion of a period. On this map he should jot down the geographical distribution of the different kinds of sediments, using the conventional signs indicated at [p. 90]: thus, in the case of the Lower Carboniferous rocks he would place the conventional sign for limestone in Derbyshire, a combination of those for limestone and shale in Yorkshire, and would add to these the sandstone sign in Northumberland. He should also note the general character of the fossils, using abbreviations for such terms as fresh-water fossils, shallow-sea fossils, deep-water fossils. After reading the account of the group of rocks in a comprehensive text-book, and inserting his notes on the map, he should proceed to insert the probable position of the coast-lines. He should also take notes of any indications of contemporaneous volcanic action, though these might well be inserted on a separate map. If this course be pursued, the student will not only have the significance of the variations amongst the strata impressed upon his mind, but he will have a means of obtaining at a glance the distribution of sediments and faunas of different kinds in the British area during the principal geological periods. On another set of maps he may indicate the axes of the orogenic movements which have occurred at different times, and when his various maps are completed, he will have the materials for the construction of a general account of the various geological processes which have been concerned with the building of the British area.