Chapter 26 Detail of the Red Measures

On account of the importance of the Mainland sections, a few notes on the best that are at present known are given below. But, as that district is not in Anglesey, it has not been mapped by the present writer, and its structures yet remain to be worked out:

1 Mainland

On the shore north-east of Carnarvon, red sandy marls, like those of the opposite coast, are obscurely seen; but at the new Brickworks near Griffithis Crossing there is a fine section, showing 50 to 60 feet of evenly bedded red marls, with grey-green bands, and a few gritty and fine pebbly seams. Along the shore overhung by trees, beyond Llanfair-is-gaer Church, the great keratophyre-conglomerate (E10565); (Plate 28), Fig. 5 is well exposed, evidently emerging from below and faulted up against the marls. Towards Port Pinorwic it reappears among the limestones, but they are faulted against and even thrust over it.

2 Foel Ferry

The only section known in this district is that which runs along the Menai shore for about a third of a mile from the Ferry, rising, under the drifts, to a height of some 10 feet. Red marls with thin pebbly sands, like those of the Brick-works, but without their clear even banding, are the only rocks visible. The bedding, indeed, is for the most part obscure, and as the cliff is a strike-section, the low dip of some 5° south-east is inconspicuous. Thin calcite-veins, often at low angles, are abundant.

3 Holland Arms

1. The New Cutting on the Red Wharf Branch railway., This is now turfed over. The rocks exposed in it were deep-red but green-mottled sandstones and marls, with calcareous geodes and concretions, the sandstones being micaceous and rather fine. Some were yellow, some even white. There were also blue-grey marls or shales. The dip was 20° to 25° south-east, changing locally to south-south-west.
2. The cutting north of the bridge north of the Station, 83 yards in length. This section has deteriorated of late years very much.
3. The cutting from the Station south-westwards to the stream, 200 yards in length.

The succession seen in the two latter will now be given, but without measurements of the divisions, as the dips are too variable and the exposures not good enough. They are only a yard or two in depth, the upper parts of the cuttings being, apparently, in drift. Each is given from north to south.

b.

Thick-bedded sandstone

Yellowish sandstone

Soft red manly beds

Red sandstone

Red marls with thin sandstone (an inch or two being pebbly)

Flaggy grey micaceous sandstone

Red marls with red sandstone

Greyish sandstone (nine inch)

Red marls with thin mica-seams

Greenish white pebbly sandstone (six inch)

Red marls, with a green bed

Compact grey-green shale

c.

Dyke

Pebbly sandstone with limestone fragments

Yellowish sandstone

Red marls

Grey shales

(Fault?)

Red sandstone, well bedded

Yellow sandstone, passing into dolomitic breccia

Mottled red and green marls, with sandy beds

(Bridge)

Grey flaggy shales

Obscure space, apparently the same

Grey shale, baked and spotted

Dyke

Obscure: shale and ochreous matter, close to the main Berw fault

In (b) the dips are 25° south-east, and 50° south-south-east; in (a) they vary a good deal, but are chiefly 20° to 25° north-west, that is to say, away from the great fault.

4 Llangaffo section

Fine red sandstones and grey-green marls, visible in a small roadside section under a hedge on the north side of the railway, 50 yards from the alluvium, and opposite a little farm, dipping gently to the north-west.

5 Berw shafts material

This is chiefly, at any rate what now remains in the walls along the road to the old engine chimney, the massive red sandstone with secondary quartz, rather coarse, but seldom pebbly. Some is finer, with mica-partings.

6 Morfa-Mawr shafts material

Much of this is of the same type; but a great deal is pebbly, and most of the pebbles examined were obtained at this place.

7 The shaft and boring records

None of the cores from the old borings appear to have been preserved, and those from the recent ones have not yet been seen by a geologist, but from Ramsay's description and the records that have been kept, it is evident that the rocks passed through were of the same character as those now visible. Time after time they record red sandstone, with conglomerates and marls, and often with considerable thicknesses of blue shale', evidently the kind of shale that is exposed in the railway cuttings. Sandstone with blue shale' is the Prevalent record in the north-eastern, Sandstone with conglomerate and marl' in the south-western borings. Abstracts of 15 records will be given here, with detail of 'C', 'L', and 'O'. The details of the others may be consulted in the offices of the Geological Survey. In the abstracts, all but the marls may be taken to be red sandstone. Dip is only recorded in 'M', 'N', and 'O', so that in all the other borings the measurements are of depth, not of thickness, but the difference would not be great. A more serious drawback is the uncertainty that prevails as to the real position of the base of the formation in eight of the borings, which has now to be inferred from the verbal descriptions.

The small depths in E. and G. are due to the positions being nearer to the outcrop of the base. The first six are all grouped around the old workings to the north-Vest of Plâs Berw. The Morfa-mawr borings have not been located with precision, but lie to the south-west of the Berw group. 'M' is situated 300 yards to the NNW., 'N' 1,000 yards to the east of Pen-y-bont, and 'O' (the Llangaffo deep boring) to the NW. of Tyddyn-isaf.

'C'. Berw Boring No. 1, 833 yards west of Plâs Berw House

'L'. Hendregadog Boring no. 2, 600 yards north-north-west of Hendregadog House

'Rock' is certainly sandstone. 'Floor', a marl or shale.

'O'; Llangaffo Deep Boring. In the Malldraeth Marsh, about 60 yards to the south of the main line of Railway, and 314 yards from the point where it enters the Alluvium.

The slides from the Red Measure pebbles are (E10548) [SH 474 645], (E10549) [SH 463 725], (E10550) [SH 455 715], (E10551) [SH 455 715], (E10552) [SH 455 715], (E10553) [SH 455 715], (E10554) [SH 455 715], (E10555) [SH 236 838], (E10556) [SH 251 806], (E10557) [SH 236 816], (E10558) [SH 437 936], (E10559) [SH 448 938], (E10560) [SH 461 936], (E10561) [SH 461 936], (E10562) [SH 438 936], (E10563) [SH 260 770], (E10564) [SH 445 813], (E10565).