September 19, 1895] 



NA TURE 



491 



can hardly feel certain ; but such amounts have been recorded 

 with certainty as occurring in the neighbouring county of Essex. 

 These great thicknesses (chiefly consisting of Boulder Clay) 

 show the importance of the Drift, and the impossibility of map- 

 ping the formations beneath with any approach to accuracy, on 

 the supposition that the Drift is stripped off, as is the case in the 

 ordinary geologic map. The records also show the varying 

 thicUness of the Drift, and how difficult it often is therefore to 

 estimate the thickness at a given sp)ot. Sometimes the sections 

 seem to point to the existence of channels filled with Drift, such 

 as are found also in Essex and in Norfolk : and it may he noted 

 that in the northern inland part of the former county, one of 

 these channels has been traced, though of course not continu- 

 ously, for some eleven miles along the valley of the Cam, and 

 at one place to the depth of 340 feet (or nearly 140 below sea- 

 evel), the bottom of the Drift moreover not having been reached 

 even then. A channel of this sort seems to occur close to us, in 

 the midst of the town of Ipswich, where, by St. Peter's, one 

 boring has pierced 70 feet of Drift, and another 127, in grotmd 

 but little above the sea-level. 



As the Drift sands and gravels, that in many places occur 

 below the Boulder Clay, often yield a fair amount of water, the 

 proof of their occurrence and of the thickness of the overlying 

 clay is of some practical good. 



The Crag. 



On this geologic division we have a less amount of informa- 

 tion, as would be expected from the fact that it is not iiearly so 

 widespread as the Drift, and this information is confined to the 

 Upper, or Red, Crag, the Lower, or Coralline, Cr.ag occurring 

 only over a very small area, and no evidence of its underground 

 extension being given by wells. 



VNTiat we learn of the Red Crag, however, is of interest, 

 several wells having proved that it is far thicker underground 

 than would have been supposed from what is seen where its base 

 crops out. One characteristic, indeed, of this sandy deposit, in 

 the many parts where it can be seen from top to bottom, is its 

 thinness, as in such places it rarely reaches a thickness of 40 

 feet. But, on the other hand, wells at Hoxne seem to prove 

 more than 60 feet of Crag, whilst at Saxmundham the forma- 

 tion is 100 feet thick, and at Leiston and Southwold over 140. 

 Further north, just within the border of Suffolk, there is, at 

 Beccles, a thickness of 80 feet of sand, or, with the overlying 

 Chillesford Clay, a total of 95. Our underground information 

 has, then, trebled the known thickness of the Upper Crag of 

 Suffolk. 



It has also shown that at some depth underground the colour- 

 name is a misnomer, the shelly sands being light-coloured and 

 not red. This is the case too with some other deposits, which 

 owe their reddish-brown colour at the surface to peroxide of iron. 

 Presumably the iron-salt is in a lower state of oxidation until it 

 comes within reach of surface-actions. This seems to point to the 

 ri.sk of taking colour as the mark of a geologic formation. 



Eocene Tcrtiarics. 



Below the Crag there is a great gap in the geologic series, 

 and we come to some of the lower of the Tertiary formations, 

 about which little had been published, as regards Suffolk, before 

 the work of the Ceological Survey in the county. It seems as 

 if the special interest in the more local Crag had led observers to 

 neglect these beds, which had been amply noticed in other 

 pirts. 



We have records of more than forty wells in Suffolk that are 

 partly in these deposits, and of these thirty six reach down to 

 the Chalk, twenty giving good sections from the London Clay to 

 the Chalk. The thickness of the Lower London Tertiaries 

 (between those formations) thus proved varies from 30 to 794 

 feet, the higher figure being much greater than anything shown at 

 the outcrop. Tlie greatest recorded thickness is at Leiston, 

 where, miireover, the top 26 feet of the 79^ may belong to the 

 uppermost and most local of the three divisions of the series, 

 the tJIdhavcn Beds, of very rare occurrence in the county. The 

 next greatest thickness is at Southwold, where the whole has 

 liten classed as Reading Beds (the persistent division), though 

 lure and elsewhere it is possible that the underlying Thanet 

 I'.cds are thinly represented. It is noteworthy that at both these 

 I'l.ices, where the Lower London Tertiaries are thick, they are 

 also at a great depth, beginning at 2524 and 218 feet respec- 

 lively, which looks as if, like the Cr.ig, they thickened in their 

 uTidergroand course away from the outcrop. 



NO. 135 I, VOL. 52] 



The important evidence given by these wells, however, is not 

 as regards thickness ; it is to show the underground extent of 

 the older Tertiary beds, beneath the great sheet of Crag and 

 Drift that prevents them from coming to the surface north-east- 

 ward from the neighbourhood of Woodbridge. It is clear that 

 over this large tract we can know nothing of the beds beneath 

 the Crag otherwise than from wells and borings ; and, until 

 these were made, our older geologic maps cut off the older 

 Tertiary beds far south of the parts to which we now know that 

 they reach, though hidden from our sight. No one, for instance, 

 would have imagined many years ago that at Southwold the 

 Chalk would not be touched till a boring had reached the depth 

 of 323 feet, or .some 280 below sea-level, nor that at Leiston 

 those figures would have been about 297 and 240. 



It is from calculations based on the levels of the junction of 

 the Chalk and the Tertiary beds in many wells that the line 

 engraved on the Ceological Survey map as the probable boundary 

 of the latter beds under the Crag and Drift has iK-en drawn. 

 From what has gone before, however, as to the great irregularity 

 in the thickness of the Drift, it is clear that this line must be 

 taken only as approximate, and open to correction as further . 

 evidence is got ; albeit the junction of the Chalk and the Tertiary 

 beds is found to be here, as elsewhere, fairly even, along aa 

 inclined plane that sinks towards the coast. 



Cretaceous Beds. 



Though the Chalk is reached by verj' many well.:;, yet we get 

 less information about it, by reason of its great thickness. More- 

 over, the great amount of overlying beds in many cases is a bar 

 to deep exploration. 



Of our Sufiblk wells there are forty which go through 100 feet 

 or more of Chalk. Of these twenty go through 200 feet or more, 

 half of these to 300 or more, and again half of the ten to 400 or 

 more, a very exact piece of geometric progression, or more 

 strictly, retrogression. Although two wells pass through the 

 great thickness of more than 800 feet of Chalk, yet neither of 

 them gives us the full thickness of the formation ; for the 816 

 feet at Landguard Fort do not reach to the base, whilst the S43 

 (or 817) feet at Combs, near Stowmarket, do not begin at the 

 top. 



As in no case yet recorded has the Chalk been pierced from 

 top to bottom in Suffolk (a defect that will be supplied during 

 this meeting by the description of the Stutton boring), that is to 

 say, no boring has gone from the overlying older Tertiary beds 

 to the underlying Cault, we must now, therefore, cross the 

 border of the county to get full information as to the thickness 

 of the Chalk ; and we have not far to go, for the well-known 

 Harwich boring passes through the whole of the Chalk, proving 

 a thickness of S90 feet. It is almost certain, indeed, that this 

 should be given as a few feet more, for the 22 feet next beneath, 

 which have been described as Gault mixed with Greensand, is 

 l)robably in part the green clayey glauconitic base of the Chalk 

 Marl. We may fairly add to this number 5 feet (as also in the 

 case of the Combs boring), and may say that, in round nundiers, 

 the Chalk reaches a thickness of about 900 feet in the south- 

 eastern part of Suffolk. Toward the northern border of the 

 county it is probably more, as the deep boring at Norwich passes 

 through nearly 1 160 feet of Chalk, and that without beginning 

 at the t<ip of the formation. 



Of our recorded Suffolk wells only three reach the base of the 

 Chalk, at Mildenhall, Culford and Combs; consequently we 

 have little knowledge of the divisions of the Chalk. These 

 divisions, indeed, are of comparatively late invention, having 

 been evolved since the publication of many of the deep sections 

 that have been referred to. 



If the Upper Chalk at Harwich goes as far down as the flints, 

 then we must allow it to be 690 feet thick, leaving little more 

 than 200 for the Middle and Lower Chalk together. .Vt Land- 

 guard Fort, from the same point of view, th Upper Chalk 

 would certainly be 500 feet thick, and one cannot y how much 

 more. 



At Combs, on the other hand, flints have been recorded as 

 present only in the top 27 feet of the Chalk ; but whilst this 

 may have been owing in part to the boring having passed be- 

 tween fairly scattered nodules, and in part, perhaps, to insuffi- 

 cient care in observation, at Harwich it is possible that some 

 flints may ha\'e been carried down in the process of borinc. 



What evidence we have tends to show, however, that the 

 Upper Chalk forms a good deal more than half, and perhaps 

 about two thirds, of the formation, the Middle and Lower Chalk 



