SECTIONAL TRANSACTIONS.—C. 395 
Llandovery (A-C). It. 
Upper Group (C) C4. ‘ Pale shales.’ Olive-green, mottled mud- 
stones, with M. crenulatus é ‘ 800 
C3. Pale olive-green sandy mudstones ; ; 80 
C2. Conglomeratic grit and limestone with 
Pentamerus “oblongus 5 : , 25 
Cl. Green mudstones with P. oblongus : ; 130 
UNCONFORMITY. 
Middle Group. B. Pale greenish-grey mudstones with Mono- 
graptus decipiens ; : : : 50 
UNCONFORMITY. 
Lower Group. A4. Grey sandy mudstones with shelly fossils and 
Monograptus atavus . 260 
A3. Micaceous olive-green mudstones with erit 
bands. Stricklandinia lens, Meristina 
subundata. ‘ 700 
: A2. Dark-grey mudstones with erit- bands near 
the base . : 350 
. Al. Massive, well- jointed basal grit, conglo- 
meratic towards the 8. , : 4 50 
UNCONFORMITY 
Bala. Blue-black mudstones with Mglina, Cystids, 
&e. 
In the greater part of the district the upper and lower groups are in uncomformable 
contact. Comparison with the Llandovery area proves that over 1,600 feet of strata 
are missing. In one locality about 50 feet of the missing strata (B) are exposed. 
28. Mr. Grorace Suater.—Glacial Tectonics as Reflected in Drift 
Deposits. 
Superficial disturbances attributed to glacial action are best developed in 
association with the softer Mesozoic and Cainozoic formations in Britain. An 
excellent example of this type of structure was seen during extensive excava- 
tions in the area of the Hadleigh Road, Ipswich, between the years 1900-1923. 
The disturbed deposits consisted of material derived from five local outcrops 
of Tertiary formations, each bed having a distinctive lithology. The sequence 
‘included Thanet Beds, "Reading Sands, Pebble Beds, London Clay with cement 
stone, and Red Crag. These disturbed deposits rested on an outcrop of Reading 
‘Sands of low relief, adjacent to which is a buried glacial channel. 
hd The upper surface of Reading Sands had been moulded into ridges resembling 
“roches-moutonnées ’ with four intervening spoon-shaped basins. 
Upon this rippled surface lenticles of London Clay had been stranded and 
“moulded into asymmetrical folds or ‘drumloidal curves.’ ‘Boulders’ of Crag 
shad in turn been arranged over these folds, and finally sand and gravel with 
portions of Crag and strips of London Clay’ overlain by boulder clay had filled 
in the basins and converted the central basin into the crest of a hill. The 
deposits on the exposed sides of the folds were associated with thrust-planes, 
“and those cn the leeward sides with ‘ tip-structure.’ 
"Chis type of tectonic structure was due to the combined effects of differential 
aes associated with two pressures varying in intensity and acting at 
ht angles to one another. Pressure from the north was seen to best advan- 
un in the north-east part of the area, whilst that from the west was well 
¢ displayed in the adjacent railway sections to the south-west. 
_ The outcrops of the disturbed beds are arranged at right angles to the 
resultant of the pressures on the exposed sides of the folds, but in the direction 
of the resultant on the leeward sides of the folds. 
The disturbed deposits are regarded as having formed bedded englacial 
material in a body of moving ice, the structure as now seen being the cast of 
the original structure. The beds have been preserved in the same relative 
positions as in the glacier, owing to an extremely slow rate of melting of the 
interstitial ice. This tranquil mode of deposition was due to the blanketing 
of the whole with boulder clay. 
