The key to Suffolk Geology

24

THE KEY TO SUFFOLK GEOLOGY.

BY H . E . P . SPENCER, F . G . S .

IN the now disused Chalk Pit at Bramford, formerly known as Coe's Pit, and in the adjoining Brickyard Pit a quarter mile to the south, all the geological formations of Suffolk are represented except the Coralline Crag, the interglacial Brickearths and the river Valley deposits. Between the two pits there is the most complete series of strata to be seen at any one site.

There are some seventy feet of chalk which about eighty million years ago was a calcareous ooze on the sea bottom ; the emergence of this above sea level brought to an end the Age of Giant Reptiles. The chalk zones are those of the fossils Belemnitella mucronata and Actinocamax quadratus.

Resting on the chalk is the Bull-head Bed composed of curiously shaped flints which are stained a dark green by Glauconite, (hydrous Silicate of iron and potassium). To this mineral the dirty greenish colour of the Thanet Bed, which lies on the Bullhead flints, is due. These flints are considered to be the insoluble residue left after the Solution of an unknown thickness of chalk.

The Thanet Bed is marine in origin and is devoid of fossils.

The freshwater Reading Beds are next in the series and here consist mainly of current bedded sands with minute flakes of mica and rolled pebbles of plastic clay, the latter sometimes in masses, indicating the erosion of clay beds at the time of deposition.

Under Ipswich the upper part of the Reading Beds are mainly green and red mottled plastic clay which is again unfossiliferous.

Normally the London Clay lies on the Reading Beds with a Stratum of blue-black pebbles at the base, the Oldhaven Pebble Bed, but this formation is missing from the series. It occurs however, in the Brickyard Pit where it is about twenty feet thick.

This is the last exposure of the London Clay to be seen north of Ipswich for it dips and thickens towards Gt. Yarmouth.

The London Clay, formerly much used for brick-making, is a deposit of mud laid down in a shallow sea off the east coast of a former continent by large rivers, just as the Amazon does to-day.

Judging from the fossils, which are rare locally, the climate must have been much warmer than at present, for one of the commonest is the fruit of the Nipa Palm which now grows along the estuaries of Burma and Malaya.

There are no surviving deposits of the Miocene and Oligocene periods in East Anglia, : this country must then have been part of a continent on which lived Mastodons, Tapirs, the predecessors of the Horse and the early ancestors of mankind. Fossils of

THE

KEY

TO

SUFFOLK

GEOLOGY

25

these creatures, and the earliest flint tools of primitive men, are found together mixed up in the Basement Bed of the Red Crag, which here rests on the Reading Sands and not on the London Clay as it normally does. Actually the Coralline Crag is next in the series, but the original extent of this deposit is unknown and its only remains are three patches, which must have been islands in the Red Crag Sea ; the largest of these lies between Aldeborough and Gedgrave.

*~o ^ o o°o <s Ä «o

BOULDER

CLAY

GLACIAL GRAVELS

RED CRAG

BASEMENT

BED

/ POSITION OF \ VLONDON C L A Y / READING SANDS THANET BULL

BED

HEAD

BED

x._L

CHALK

jT.

1.

I.

U I. 3ZZIL-

u L U "jr SECTION OF STRATA AT BRAMFORD CHALK PIT.

26

THE KEY TO SUFFOLK

GEOLOGY

The Red Crag occupies an area from Walton-on-Naze to Sudbury and eastward to Needham Market, thence northward to Norwich.

North of Aldeburgh it differs in character and is known as Norwich Crag ; it contains a greater proportion of shells which exist in northern waters to-day. While the Coralline Crag was deposited in a sea as warm as the Mediterannean (and is now our only Pliocene formation), the Red Crag sea became increasingly cooler, and as the first arctic foramnifera appear in it at Walton and the fossil remains of elephants and horses occur in South-east Suffolk, so the deposits laid down by it are at present regarded as lower Pleistocene. The Crag Sands are normally very rieh in fossil shells, but it is here without shells and those fossils in the Basement Bed are chiefly derived from older deposits. In this pit, however, the bones of bear, antlers of an extinet species of deer, Cervus falconeri, and the teeth of beaver are contemporary with the deposition.

Above the Red Crag are sands and gravels variously known as Westleton Beds or Mid Glacial Gravels. They contain many erratic pebbles derived from some older deposit of glacial origin and as they are overlaid by the Upper Chalky Boulder Clay-the term inter-glacial may be with good reason be applied to them.

T h e Upper Chalky Boulder Clay Covers much of mid-Suffolk and is about 100 feet thick in places ; under the adjoining Whitton Estate it is about fifteen feet thick.

FROST

CRACKS.

FOR the past three years several members of the staff of the National Agricultural Advisory Service have been trying to solve a puzzle in various crops at Boyton Hall, near Woodbridge, which is managed

by Mr. A. G . Jaques. In several fields on the farm, crops of cereals and lucerne in particular have shown distinet lines of plants which have stood higher and appeared more thnfty than the plants over the remainder of the fields. Analyses of plants and soil have not solved the puzzle.

In August, 1952, an undulating field of lucerne facing the River Ore showed very distinet lines varying in width from 34-42 inches of lucerne which was distinctly greener and taller than the plants outside the lines. These lines were distinctly straight and turned off at marked angles and joined up to form a series of polygons. T h e soil is a deealeified red crag over red crag and is subject to effects of drought.

A trench was dug to a depth of 4 feet across one of the lines of tall lucerne and it was seen that the depth of deealeified crag under