Frost Cracks

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

FROST CRACKS

27

the " line " was much deeper than outside the " line," where it changed to red crag proper at 20 inches. A deep boring auger was then used to a depth of 110 inches in bores on the " line " and outside the " line." Outside of these lines of flourishing lucerne, the soil was recorded as follows :— to 20 ins.

Decalcified red crag to 47 ins.

Red Crag very dry and thereafter increasing in moisture at 67 ins. v. moist light beige crag at 73 ins. v. moist iron discoloured at 80 ins. dark red and brown at 90 ms.

Grey-green sand at 95 ms. with small flint and coprolite at 100 ins.

Sand and London clay at 105 ins.

London Clay When the auger was put down over the " line " of tall lucerne, it was found that the depth of decalcified crag was 57 inches and at this depth the red crag proper was Struck. Mr. H. E. P. Spencer, F.G.S., the geologist at the Ipswich Museum, came to the conclusion that these " lines " were frost cracks which occurred in the last phase of the Ice Age.* In the accompanying diagram of the soil it will be seen that the lucerne roots had the advantage of 57 inches of comparatively loose light soil over the frost crack whilst off this line, the hard shelly red crag was Struck at 20 inches and this was particularly dry to a depth of 48 inches, whilst the moisture was considerable near the surface and more uniform throughout the decalcified crag in the frost crack.

At the lower end of the field near the River Ore, at the same date in August the lucerne was flourishing and gave no sign of suffering from lack of moisture. The first 55 inches of soil comprised a sand which was not of crag origin, under which the crag became increasingly moist with iron discolouration ; at 78 inches some flints were found and particles of London clay.

Lucerne is one of our most drought resistant plants on account of its deep rooting system, which can penetrate to a depth of at least 15 feet. It does, however, like a pervious soil and the red crag bed found in this instance at 20 inches proved a limiting factor in the plant's search for moisture. P . J . O . T R I S T . * T h e frost cracks were probably formed during t h e last glaciation, perhaps 50,000 years ago, when it is believed the Ice C a p did not actually reach this area w h i c h was covered only with snowfields. Fluctuations in the intensity of the cold would cause thawing w h i c h would saturate sandy soil which might then have a tendency to creep downhill and crack. W i t h an increase of cold the cracks would fill with snow which would compact into ice, widening the crack by wedge action. I n post glacial times the action of the weather on the soil would penetrate deeper along the lines of the cracks, thus providing narrow zones of deeper sub-soil.

H.E.P.S., October, 1952.

FROST CRACKS

28 BOYTON

HALL,

NR.

WOODBRIDGE.

O.S. 509/143.

Inches.

0

A

10 15

B

Frost Crack

5 Decalcified Red Crag

20

Decalcified

Sand

Red Crag

(Not of Crag Origin)

25 30

Red Crag

35

(Dry)

40 45 .50 55

Increasing Moisture

60

Increasing Red Crag

•65 70

Light

Beige Crag v. moist

Moisture with Iron Streaks

75 Iron Discoloured 80 85 90 95

Dark Red and Brown Crag Grey Green Sand

100

Sand with Flint and Coprolite

105

Sand and L o n d o n Claj

Flints and Clay Particles London Clay

London Clay A.

Bore 44 ins. off line of frost c r a c k : luceme 4 - 6 ins. high.

B. C.

Bore through frost c r a c k : luceme.

Bore at lower end of field near river.

August, 1952.