| Ins. per second. | Mile per hour. | ||
| 3 | = | 0.170 | will just begin to work on fine clay. |
| 6 | = | 0.340 | will lift fine sand. |
| 8 | = | 0.4545 | sand as coarse as linseed. |
| 12 | = | 0.6819 | will sweep along fine gravel. |
| 24 | = | 1.3638 | will roll along rounded pebbles 1 inch in diameter. |
| 36 | = | 2.045 | will sweep along slippery angular stones of the size of an egg. |
These figures[2] have greater interest for us than in the connection at present used, as will be noticed hereafter. We have seen that in rounding the bends ([Fig. 1]) A, B, C, different portions of the stream possess different velocities. We know it is charged with sediment and stones all the time. The tendency, therefore, will be for the large stones and coarse detritus to go round the outer side of the bend, to bombard the banks near the points shown by the arrows, and to erode the channel deepest in those situations; whilst a goodly proportion of the fine muddy sediment will find its way to the quiet and shallow parts near x x x, and in course of time become deposited there, whilst the main course of the stream is eating its way and shifting its course as indicated by the dotted lines a a. This action proceeds, it may be, until the course of the river becomes straighter, as shown by the dotted lines b b, when the whole of the loop B D is abandoned, its former course there being evidenced by pools of water and irregular heaps of gravel, sand and mud. The reader will now see that the whole of the land marked x x x has been formed of sediment brought down by the river, and in the majority of cases such fine silt and sandy mud or clay is specially suitable for brickmaking—many of our largest brickmakers obtain their material from such a source. It should be observed that the valley, as shown between the lines v v, may be two or three miles in width, and it is often much more, so that the actual amount of land made by the river at x x x may be several thousands of acres in extent.
Now as to the practical application of the foregoing observations. In the first place, it will be seen that such deposits of brick-earth as are made in this manner cannot be very thick, their total thickness perhaps, resting on the bottom of the valley, not being more than 20 feet, and it is frequently much less. The next thing to be noticed is that they must be very variable in character, a bed changing perhaps every 100 feet or so horizontally, and more often every few feet. Individual beds must of necessity be very irregularly developed under the circumstances. The velocity of the stream being greater at certain seasons of the year than at others, we frequently find some such section as the following developed:—
Fig. 2.—Section of Fluviatile Brick-earth.
a = Mould and soil, of no use to the brickmaker.
b = Sandy clay, with a large proportion of sand; useful for moulding or incorporating with the “fat” clays below for brickmaking.
c = Gravel bed, lenticularly developed; suitable for mending roads, paths, &c.
d = Sandy clay; similar to b.
e = Thin bed of marl, with a fair proportion of lime.
f = Sands and small pebbles, irregularly stratified (false-bedded).
g = Laminated sandy clay.
h = Stiff clay; can be mixed with f and passed through the pug mill.
i = Sand; an irregular bed of very local occurrence.
j = Gravel bed, with much sand.
The above is typical of deposits accumulated in river valleys; it is different in character to deposits laid down in the sea (as will presently be described); the section exhibits very different classes of brick-earth also, and yields a totally different kind of brick to that obtainable from brick-earths of marine origin. The importance of the question of origin of a brick-earth, therefore, is just beginning to dawn upon us. Many rivers are noted as having throughout a long period of time wandered from one side of the valley to the other (by the process depicted in [Fig. 1]) several times, in which cases the brick-earth sections relating to them are liable to still greater variation. The reader would perhaps be very much astonished to find how much is known concerning peregrinations of that description in regard to particular localities, by competent authorities—usually field geologists.
We come to another important point in regard to river deposits. The ceaseless flow of the river, and the abrading action of the large stones rolled along at the bottom of its channel, tend to cut the latter deeper and deeper, and we have excellent evidence that most English rivers once flowed at a greater elevation in their valleys than they now do. In consequence of this, the brickmaker may find his pit somewhat higher than the neighbouring river, which at an earlier stage of its existence made his brick-earths. To a certain extent, small earth movements, as previously explained, are also undoubtedly responsible for many of these brick-earths now being at a considerable elevation above the surface of the river. This phenomenon is illustrated in [Fig. 3].
Fig. 3.—Section across a river valley, showing formation of terraces of gravel and brick-earth.