The Geological Interpretation of a Gravity Survey of the English Lake District and the Vale of Eden

The geological interpretation of a gravity survey of the English Lake District and the Vale of Eden

MARTIN H. P. BOTT

CONTENTS Rock densities and methods of survey . . 3Io Interpretation of the Lake District negative gravity anomaly 313 (x) Eskdale granite and Ennerdale granophyre 3x3 (n) Skiddaw granite 3t5 (e) Shap granite 3x7 (I)) Hidden granite beneath the Lake District 3~8 (~) Other gravity anomalies 3x9 Uplift of the Lake District 320 Vale of Eden . 321 (A) Granite ridge at "depth joining Shap and Weardale granites 32x (13) The Permo-Triassic trough . . . 322 (c) Structural history of the Pennine fault line 324 References . 326

SUMMARY The Lake District is dominated by an east- is probably connected to the Weardale granite west belt of relatively low Bouguer anomaly by a deep granite ridge. which attains individual minima over the Present and past uplift of the Lake District exposed Eskdale, Shap and Skiddaw granites. may be attributed to the granite mass deficiency, The negative anomaly is attributed to a which is estimated to be x.x × xolSg and is composite granite batholith which underlies approximately equal to the present elevation of the central and northern parts of the Lake the Lake District above a 27 °ft (82 m) datum. District, connecting the exposed granites at The low gravity values along the Vale of depth. Interpreting on the basis of surface Eden suggest that the Permo-Triassic rocks density measurements, the granites appear to reach a maximum thickness of at least 1 km extend to a depth of about 7 to xo km and the northeast of Penrith, and that these rocks contacts with the country rocks generally slope formed during contemporaneous subsidence. outwards. There are substantial variations in The detailed gravity interpretation of the density within the composite granite body. The structure of the Vale of Eden allows a new roof region of the granite includes a series of assessment to be made of the structural history shallow granite 'ridges,' one of these connecting of the Pennine line which reconciles the the Eskdale and Shap granites, and another Hercynian structures with the occurrence of connecting the Ennerdale, St. John's, Threl- Whin Sill or dyke pebbles in the Upper keld and Skiddaw granites. The Shap granite Brockram.

TH E L A ~p. D I ST R I CT is formed of Lower Palaeozoic rocks which were strongly folded in late Silurian or early Devonian time, and were intruded by post tectonic granites of early Devonian age. Faulted and tilted Carboniferous strata overlie the earlier beds with strong unconformity; these generally dip away from the Lake District and their outcrop forms a discontinuous belt around it. The Carboniferous rocks are unconformably overlain by Permo-Triassic beds, which are faulted against the Lower Palaeozoic rocks along the western margin of the

Jl. geol. Soc. Lond. vol. x3o, I974, pp. 3o9-331 , 8 figs. Printed in Northern Ireland.

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Lake District. The region suffered earth movements in Hercynian and post Triassic times which caused doming of the Lake District and faulting and tilting in the surrounding regions. General accounts of the stratigraphy and structure are given by Hollingworth (i954) , Mitchell (I956), Taylor et al. (1971) and Moseley (i972), with lists of references. This gravity survey was made primarily to investigate the deep structure of the Lake District granites and of the Permo-Triassic trough of the Vale of Eden.

I. Rock densities and methods of survey

Densities measurements on Lower Palaeozoic and Permo-Triassic rocks are shown in Table I. The densities of these formations at depth probably lie between the saturated and grain density values. The density of Carboniferous formations is based on measurements in NE England (Bott & Masson-Smith I957) , and this depends critically on the propor- tions of limestone (mean saturated density 2.68 g cm -3, mean grain density 2"7 I), shale (2.56, 2"69) and sandstone (2"42, 2"65). A typical Yoredale sequence has a density of about 2.60 g cm -8. Significant density contrasts occur at the following interfaces: Ordovician/ Silurian, Lower Palaeozoic/granite, Lower Palaeozoic/Carboniferous, Carboni- ferous/Permo-Triassic. Although some density variation occurs within formations, the main gravity anomalies can essentially be interpreted in terms of the above density contrasts. The survey of 874 new gravity observations was made using Worden gravimeter No. 138. The calibration factor was verified by measuring between pendulum stations at York and Newcastle-upon-Tyne. The calibration error is probably less than +o. 1%. The local base network (Fig. I) is connected to I.G.S. gravity base stations at Kendal, Shap and Penrith and is related to a value of 98I'265oo cm s -* at Pendulum House, Cambridge. The maximum closing error within the base network is 0.04 mgal. During the survey, base connections were normally made at not more than two hour intervals. The standard error of a single gravity obser- vation is estimated to be +0.05 mgal. Correction for latitude was made using the I.G.F. and the combined elevation correction was applied using the following formation density values: 2"75 g c m-3 (Skiddaw Slates and Borrowdale Volcanics), 2"72 (Silurian), 2"65 (granite and Carboniferous except Coal Measures), 2.50 (Coal Measures), 2.33 (Permo- Triassic) and 2.oo (alluvium). Terrain corrections were applied using the Hammer zone chart method. The largest unsystematic source of error in the Bouguer anomalies comes from the terrain corrections, and may be as much as +0"5 mgal in regions where the corrections approach i o mgal, although more usually this error will be about 4-o-15 mgal. A small systematic error is introduced by uncertainty in rock density, but is unlikely to exceed zko.5 mgal even at elevated stations.

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Saturated density Grain density Locality (grid Number of with s.d. with s.d. Formation reference) specimens (g cm -s) (g cm -3) Source

Skiddaw Slates NY218277 to 2"78 -4- 0-02 2.8I 4- 0.02 (i) NY217279 I9 2"78 4- 0-02 2.8I 4- 0.02 NY244236 i7 2"76 4- o-o2 2.78 4- o.o2 NY2419 7 2.76 4- o.oi 2"77 4- o-o2 Mean of localities 2"77 4- o.oI 2"79 4- o-o2 Borrowdale NY244135 20 2"76 4- 0"03 2"77 4- 0.02 (I) Volcanics NY216o72 I t 2"80 4- 0"02 2"8I 4- 0"02 NY2 i8o74 2 2.8o 2.82 NY22oo76 24 2"70 4- o.03 2"70 4- o.o2 NY222o95 22 2"77 4- 0"03 2"77 4- 0"03 NY22IO 2I 2.7i 4- 0.02 2.71 4- 0.02 NY253167 24 2.76 4- o.oi 2.78 4- o.o2 NY249199 21 2"76 4- o.oi 2"79 4- 0.02 SD29698o 2I 2"69 4- 0-02 2"7 ° 4- 0"03 SD28199I 24 2.66 4- o-o3 2"67 4- o'o3 SD29o98o 36 2"75 4- o.o2 2"79 4- o.o2 SD278982 2i 2.78 4- o.oi 2.78 4- o.ol Mean of localities 2"74 4- o'o4 2"76 4- o-o5 Brathay Flags SD299975 20 2"74 4- o.oi 2"77 4- 0.02 (I) NY358oI6 2I 2.75 4- 0.02 2"77 4- o.oI Bannisdale Slates 2I 2"72 4- o-oi 2"73 4- 0.02 (I) Kirkby Moor Flags SD5892 20 2-69 4- 0.02 2"74 4- o.o1 (x) Penrith Sandstone NY54231o 2I 2"40 4- 0"05 2"63 4- o.oi (i) NY528493 I3 2"44 4- 0"05 2.64 4- o.o1 St. Bees Shale NY485673 13 2"45 4- o'o4 2"77 4- o'o3 (I) NY4565o9 4 2"47 4- o-o5 2.71 4- o.o2 St. Bees Sandstone NY46o534 2I 2.26 4- 0"03 2.61 4- o.oi (1) NY528587 I6 2.28 4- 0"05 2-62 4- 0.02 NY527589 I3 2.29 4- 0-06 2.62 4- 0.02 NY61°372 3 2.I2 4- o'o3 2-63 4- o.o2 NY613326 4 2.I 5 4- 0.08 2"64 4- 0"03 Mean of localities 2-22 4- o'o7 2.62 4- o.oi Eskdale granite NYI49O5O 5 2.64 4- o.oi (2) NYI22oi 3 6 2.63 4- O'OI NYI9x°I3 9 2.6I + o.ox NY162oo3 9 2.62 4- o.oi NYI47OO4 5 2-62 4- o.o1 SDx6199o 6 2.6I 4- o.oI $D17o979 4 2.63 -4- o-o2 SDI I2943' 7 2'69 4- 0.02 SDI339o7" 8 2.73 4- 0.03 Ennerdale NYI i2152 5 2-63 4- o.oI granophrye NYI24145 9 2.62 4- 0"03 Shap granite NY557o84 9 2.66 4- o-oi (2) Skiddaw granite NY3233 8 2.58 4- 0.02 2.63 + o.oi (I) Threlkeld NY328243 I7 2"67 4- 0"03 2.7 ° 4- o-ol (,) microgranite

Sources of data: (I) Collected by Bott and Hadand, measured by Masson-Smith (I 958). (2) Collected by Bott, measured at Durham. * graxtodiorlte 2

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3O 2 3 I I

......

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-,~-,- Base of Borrowclale Volcanos • G,av,t~ t~ase I I 3 4 FIo. z. Bouguer anomaly map of the English Lake District and the Vale of Eden with gravity contours at one milligal interval. The density values used in applying the Bouguer correction are in the text. EN-Ennerdale granophyre, ES-Eskdale granite, SH-Shap granite, SK-Skiddaw granite, ST-St• John's microgranite, TH-Threlkeld microgranite. Positive closures are marked by hachures, negative closures are unmarked.

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2. Interpretation of the Lake District negative gravity anomaly A conspicuous belt of low Bouguer anomaly (Fig. x) follows the main axis of the Lake District, with minimum values over the Eskdale and the Shap granites. An offset circular minimum occurs over the Skiddaw granite region. The geology shows that this composite negative anomaly cannot be caused by low density sediments. Limiting depth criteria show that the source of the negative anomaly locally lies within, at the most, a few kilometres of the surface. The density measurements show that the granites are significantly lower in density than the intruded country rocks of Ordovician (and locally Silurian) age, and there is a close correspondence between the minimum anomaly regions and the outcropping granites. These factors indicate that the granites themselves are the principal source of the low gravity values. The anomalies indicate that granite is much more extensive at depth than at the surface. Table i shows that the density of the granite varies from one granite mass to another. It also varies within the Eskdale granite. In making interpretations, the Skiddaw Slates and Borrowdale Volcanics are assigned a density of 2"75 g cm -3 and the Silurian a density of 2.7 r g cm -s.

(A) ~.SKDA.LF. GRANITE AND ~-NNERDALE GRANOPHYRE The lowest Bouguer anomaly values of the Lake District occur over the outcropping Eskdale granite. This region of low gravity encompasses the Ennerdale granophyre and extends eastwards beyond the granite outcrops towards the Shap granite. Westwards, the anomaly merges with the low gravity anomaly produced by the east Irish Sea sedimentary basin (Bott I964). The Eskdale granite is the largest exposure of igneous rock in the Lake District. A coarse perthitic muscovite granite, dated at 383 4- 2 m.y. (Miller 1962), occurs in the north and a biotite granodiorite with abundant basic clots and xenoliths in the southwest (Simpson i934). Dwerryhouse (I9o9) , Green (1917) and Simpson (I 934) considered the Eskdale granite to be a laccolith, but Trotter et al. (1937) suggested a stock-like form with a flat concordant roof and steep discordant walls. The Ennerdale granophyre, dated at 37 ° m.y. (Brown et al. 1964) , is a pinkish granophyre with some basic patches (Rastall i9o6 ). Eastwood et al. (I93r) and Trotter et al. (1937) suggested a stock-like form, in contrast to earlier interpretations as a laccolith. The Eskdale granite has a large variation in density (Table I ). The granodiorite is denser than the granite that forms the main outcrop. Within the granodiorite, the density appears to increase towards the south. This density pattern is con- spicuously shown up by the gravity contours (Fig. I) but is not seen well in Fig. 2 as profile AA' passes to the east of the granodiorite. The granite has a density contrast of about -o. r 3 g cm -8 with the Borrowdale Volcanics, but that between the granodiorite and Borrowdale Volcanics increases almost to zero at the southern contact. Three possible two-dimensional interpretations of subsurface shape beneath profile AA' (Fig. x) are shown in Fig. 2. The interpretations are not quite rigorous

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as the profile has one change of direction, and deviation from a true two-dimensional shape causes the depth to the base to be slightly underestimated; nevertheless the main features of the interpretations are unlikely to be affected by these simplifying assumptions. Beyond this, no single model can be regarded as unique because the density contrast may vary laterally and with depth. The negative anomaly along AA' has an amplitude of about --38 mgal and is markedly asymmetrical with the minimum occurring near the south end of the negative region. The main uncertainty of the interpretation is in the explanation of the asymmetry. In models (a) and (b) this is attributed to lateral variation of density within the composite batholith, the floor being assumed to be at constant depth. Model (a) is the simpler structure, but it shows a higher density for the Ennerdale granophyre than Table i indicates. Model (b) is consistent with the surface sample density measurements, and incorporates a relatively low density Ennerdale granophyre of i km thickness underlain by denser granitic rocks beneath. Model (c) assumes a granitic body of uniform density, and attributes the asymmetry to variation in the depth to the floor. On the basis of the measured density values, model (b) is the most acceptable interpretation of those presented in Fig. 2. All three models require the granitic rocks to extend down to a depth of about 9 km, and the contacts with the country rocks to slope outwards from the granite. The models in Fig. 2 all show a granite ridge, about 3 km northwest of the contact between the Ennerdale granophyre and the country rocks, which reaches within a few hundred metres of the surface. This occurs near the SW extremity of a belt of spotted slates described by Rose (1954) and Jackson (I 96 I) and interpreted by them in terms of an underlying granitic ridge. Further south, Folkmann (1969) examined gravity profiles across the western contacts of the Eskdale and Ennerdale granites in the vicinity of Hale and the River Esk. After correcting for the known thickness of low density Permo-Triassic and Carboniferous rocks, he found that the Ennerdale granophyre roof region extends some 2 km west of the mapped boundary, and the Eskdale granite extends 4 to 6 km westwards from the surface boundary. Figure 2 shows that the Eskdale granite along profile AA' must extend south- eastwards beyond its surface contact with the Borrowdale Volcanics for a con- siderable distance. It is possible that a denser marginal granodiorite occurs here, in which case the roof would be shallower than indicated in the models. In between the exposed Eskdale and Ennerdale intrusions is an outcrop of Borrow- dale Volcanics; the gravity interpretations imply that these rocks are underlain by granitic rocks at relatively shallow depth. Thus the Ennerdale and Eskdale intrusions appear to the gravimeter as separate phases within a single composite batholithic intrusion which underlies much of the western part of the Lake District. The Eskdale granite is batholithic in space form, but the Ennerdale granophyre may be a relatively thin upper marginal low density phase within the batholith. Assuming the floor is at constant depth, substantial and fairly systematic variations in density appear to extend in depth throughout the intrusion, as in the Criffell granodiorite (Bott & Masson- Smith i96o ).

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(B) SKIDDAW GRANITE A circular region of low gravity overlies the Skiddaw granite and its thermal aureole. This anomaly is offset to the north of the main belt of low gravity of the Lake District, to which it is joined by a trough of low gravity enclosing the outcrops of the St. John's and Threlkeld microgranites. The Skiddaw granite (Rastall i9io , Hitchen I934, Eastwood et al. I968 ) is a medium grained biotite granite dated at 399 4-6 m.y. (Miller i962 ) and em- placed on the main anticlinal axis of the Skiddaw Slates. A thermal aureole encompasses three outcrops interpreted as isolated exposures of a stock-like body. The Skiddaw granite cuts across the older Carrock Fell Complex at relatively

,al ,- BACKGROUND

10 A (NW) (SSE) A (al O ~,,~// -"C.--.s'// /f\ I ~/\ ~/// :,':C,','.., .' ,,,, /i ,,.\

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27s /,:;,';26¢';:~0~2~,~ 2s3~ 26~ ',"~,2d; ,~ \ 27s /~",, '-'.~!,-, ,'.,¢/_.,,. ,/:,', ; ",- ,5 . . ;,, .-., -,. \ /.V.'..',L~.:' ".'57.:,.~ ./-, 5 , ~.,,, Y. ~ ." . ~. / ~ .. :1- ,'.\

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I I I I I ! 0 10 20 30 40 50km Fxo. 2. Two-dimensional interpretation of the Bouguer anomaly across the Eskdale granite and Ennerdale granophyre along line AA' of Fig. z, showing three possible models of subsurface structure. In Figs. 2 to 5 and 7, solid circles represent the observed Bouguer anomaly values and the continuous line is the computed anomaly for the uppermost model related to the background level shown; rock densities in the models are shown in g cm-3; for models (b) and (c) the residual anomalies (observed minus calculated) are shown above.

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shallow depth, suggesting that the Complex may not be a significant contributor to the gravity field. The Skiddaw granite has a lower measured density than the other Lake District granites (Table x). A lower limit to the density contrast with the Skiddaw Slates can be estimated by treating the granite body as a vertical cylinder of infinite depth extent with density contrast p and radius r. The gravity anomaly at the centre of the top surface of such a cylinder is 2=Gpr, where G is the gravitational constant. The amplitude of the anomaly along DD' (Fig. 3) is about -19 mgal and the appropriate radius is about 4"5 km. Substituting gives a minimum density contrast of o.io gcm -3, and it is realistically likely to be some 50% or thereabouts greater. A density contrast of --o.z 5 gcm -S has been assumed in the interpretation (Fig. 3); it would be difficult to obtain a satisfactory fit with a smaller density contrast, but if the true density contrast is larger than this then the depth extent of the model would be correspondingly reduced and the contacts with the country rocks would slope slightly less steeply outwards. The granite has been treated as a three-dimensional body of 9 km width perpendicular to the profile. The com- putations have used a technique which splits up the model into a series of hori- zontal two-dimensional prisms and applies end-corrections to approximate the three-dimensional gravity effect (Bott & Tuson I974). The interpretation (Fig. 3) shows a stock-like body of low density granite with steep nearly vertical walls and a roof region which extends well beyond the exposures and encompasses them. The contacts appear to slope outwards at shallow depth, then steepen. The depth is of the order of 6 km. This is less than indicated by the interpretations over the main granite belt, but the stock-like body may merge at a depth of a few kilometres into the northern wall of the main granite belt as shown in Fig. 6.

regal ~BACKGROUND~

D (WSW) (ENE) D*

FIG. 3" i i "" i _ "1 2.75 /- i 2-~o '.- ~ I 2.7~ Interpretation of the Bouguer anomaly iI across the Skiddaw granite along line 1 / / -- / \ \ "" I DD' of Fig. x, using a three dimensional km model of subsurface shape. ~ ,'o 15' km

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The gravity interpretation agrees well with geological inferences on sub-surface shape. The thermal aureole (Eastwood et al. 1968) is rather wider than would be expected for the gravity model. The NE margin of the aureole corresponds closely to the position of the wall, but at the SW edge the agreement is less good. The disagreement may partly stem from lack of gravity stations here, and partly from the presence of denser marginal phases or rings.

(c) S AP ORA r E The outcrop of the Shap granite covers 8 km 2 and occurs near the minimum of a much larger region of low gravity values extending over xo × 12 kmL This forms the eastern part of the Lake District gravity low. The steepest Bouguer gradients occur across the NW margin of the low, which is situated about 1o km NNW of the granite outcrop. The amplitude is about --24 mgal. The Shap granite (Harker & Marr i89I , Grantham z928 ) is a porphyritic biotite adamellite dated at 393 4- i I m.y. (Brown et al. 1964). A thermal aureole extends for some distance from the contact. Marr (I 9~6) interpreted the space form as a 'cedar tree laccolith.' The measured density (2-66 g cm -3) is higher than those of the main mass of the Eskdale granite and of the Skiddaw granite. This may reflect the magma chemistry, or contamination as suggested by basic clots. The density contrast between the outcropping granite and the Borrowdale Volcanics is estimated as --0"09 gcm -3. Profile CC' (Fig. t) is interpreted assuming a two-dimensional structure in Fig. 4. Some details of the interpreted models are uncertain because of the wide station spacing. Ambiguity is introduced because lateral density variation within

regal

C(N) (S) C' (a| 0 ======~ n _l~k ...... • ...... - ...... I~" ~ '~' I I ~ I/ ~ -- " ...... 71 ...... 5

regal *2 .gE ~/;.: . - = . ~ residuals • • • 2.69 = • " • • • (b) /:',./.,-.~.~;~.;/.,~,i:",:/..I/'l~r"~J'~"/.:.," ' ... l / ~l~l "/ " ~"~'~"..\"-..~.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.: " " - ":" :" :" : ":" ' " " ...... ' " " " ' 2"75 .~_2-66,5"2-63~I~.2,66,,/ -.2"63-- ~2"75--"~':':':':2-71:':':':':':':':': •

I I I I I 0 10 20 30 40 km F zo. 4- Two-dimensional interpretation of the Bouguer anomaly across the Shap granite along line CC' of Fig. I, showing two possible models of subsurface shape.

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the granite can be set against variation in depth to the roof, and southerly subsurface extension of the granite can be set against variation in Silurian thickness. In Fig. 4, model (a) attributes as uniform density as possible to the intrusion, and model (b) allows for more extensive lateral variation of density beneath the northern roof region. Whatever the detail of interpretation, the subsurface granite roof occupies a much larger areal extent than the outcropping Shap granite, extending at shallow depth some I o km NNW of the exposed granite. The subsurface shape is that of a fairly typical small granite batholith extending down to 8 km depth with outward sloping wails. The profile cannot be interpreted satisfactorily in terms of a granite of uniform density even if the floor depth is allowed to vary, and in particular a zone of lower density granite must occur immediately to the southeast of the outcrop to account for the position of the region of minimum anomalies; this low density phase may form an incomplete ring to occur towards the north as shown in model (b).

(D) HIDDEN GRANITE BENEATH THE LAKE DISTRICT The region of low Bouguer anomaly extends between the exposed granites indi- cating that granitic rocks must underlie much of the central and northern parts of the Lake District. To indicate the subsurface shape of the unexposed granite, a two-dimensional interpretation of gravity observations along BB' (Fig. i) is shown in Fig. 5. There is ambiguity about the shape of the roof, because the granite density distribution and the shape of the base of the Silurian are not known. However, the roof of the batholith must come within less than about i km of the surface to explain the gradients and curvature of the profile. The model shows two main granite ridges. The southern ridge underlies the trough of low gravity connecting the Eskdale and Shap granites, and is interpreted as a low density granite phase. Further north, the interpretation indicates the presence of a second composite ridge, which produces a bench of relatively uniform gravity anomaly on the northern flank of the main anomaly (Fig. I), its gravitational effect being less pronounced than

regal BACKGROUND

B (NNW} (SSE} B' 7 2.7s p,,2.7~ :," "2:~ ' , "/ " :2-6s-" " ~ \ 2-7s "~'~::2.H::::. ~" k'- ".'-~," ~ ', -~ -- /--.:- _ z \ - "- "~"-':~ /-,.....,,-..-_-.',,.~--':.-'/:.:.':- , ~/ ,-, ... , ." ".".. ". ,~~\~ 1if ......

F I o. 5- Two-dimensional interpretation of the Bouguer anomaly across the hidden Lake District granite batholith along line BB' of Fig. I.

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that of the southern ridge because of the higher density. This ridge extends from the NE end of the Ennerdale granophyre eastwards, incorporating the exposed St. John's and Threlkeld microgranites where it swings northwards towards the Skiddaw granite. The northern subsidiary part of this ridge in Fig. 5 corresponds in position to the eastern end of the belt of spotted slates interpreted by Rose (z 954) and Jackson (i96i) in terms of an unexposed granitic ridge. The model also incorporates a bench of dense granite at its northern end. A sketch map of the approximate region underlain by the roof of the Lake District granite batholith and the position of the postulated ridges is shown in Fig. 6. (E) OTHER GRAVITY ANOMALIES The Silurian rocks of the southern Lake District cause a small negative gravity anomaly because of their relatively low density as basement rocks. This appears on Fig. z as an abrupt decrease in the Bouguer gradients on passing south from

30 1 2 3 4 5 6 7

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j/ 4 , ,jii!ili.iiii ' ., ======_ x~-~-\/WEARDALE/

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~ Granite

I I I I I o ...... ~o 30 1 2 3 4 FIG. 6. Sketch map showing the inferred distribution of unexposed and exposed granite beneath NW England. The map shows the roof and wall regions of the postulated Lake District batholith, the main granite ridges indicated by the gravity anomalies, and the postulated connection to the Weardale granite.

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the Borrowdale Volcanics onto the Silurian rocks. Thick low density Silurian rocks are needed to obtain satisfactory interpretations of profiles BB' and CC'. Assuming a Silurian density of 2.7I g cm -3, a maximum thickness of 6 to 7 km is indicated in the computed models (Figs. ff and 5), but to some extent this thickness could be set against a more southerly extension of the granite batholith. In Figs. ff and 5 the Silurian rocks take the form of a trough or syncline Io to r 5 km wide, but this feature cannot be regarded as well-established as it depends on a relatively small change in gravity anomaly. The Carboniferous and Permo-Triassic rocks of the western margin of the Lake District and the Furness District cause relatively low gravity values along the coastal strip. The observed gravity anomalies on land are consistent with known thicknesses, but gravity observations at sea show that the combined thickness increases substantially towards the centre of the east Irish Sea basin (Bott I964).

3. Uplift of the Lake District

The Lake District is at present elevated in relation to the surrounding regions, and the geological history indicates that it has tended to be a positive region since the Devonian. In common with the elevated regions of the Northern Pennines and the Cheviot Hills, it is underlain by granite. Furthermore, the axis of the main granite belt approximately corresponds to the belt of highest elevations. It is therefore of interest to make use of the gravity anomalies to test whether the present elevation could be attributed to the isostatic effect of the underlying low density granitic rocks. The mass of the Lake District rocks above sea level has been estimated from topographic maps to be I "49 × 10 TM g, assuming a mean rock density of 2.74 g cm-3. This estimate relates to the regions where Skiddaw Slate and Borrowdale Volcanics occur at the surface, except that it includes the Silurian Shap Fells of grid squares NY 2450 and NY 2550 (west of grid line 256 ). The average elevation is Io35 ft (312 m). A lower limit to the underlying mass deficiency produced by the low density granites can be estimated by applying Gauss' theorem to the Bouguer anomaly map. This has been accomplished by integrating the Bouguer anomaly A(x,y) related to an assumed background value of 24 mgal over a slightly larger area than above, whence

mass deficiency = 2rrGI ffAdxdy.

Using this method, a lower limit of 7" i x IO17 g is obtained. By applying the two- dimensional version of Gauss' theorem to the interpreted anomaly profiles shown in Figs. 2, 4 and 5, it can be shown that the above method would underestimate the true mass deficiency for a two-dimensional structure by about 30%. If allowance is also made for the lack of integration to the west of the Lake District, a realistic estimate of I.I X I olSg is obtained for the granite mass deficiency beneath the Lake District.

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Thus the granite mass deficiency could be regarded as approximately compensating the present elevation of the Lake District above a datum of 27 oft (82 m). This suggests that the Caledonian granites beneath the Lake District may be responsible both for the present elevation and for the past tendency for the region to be uplifted in relation to its surroundings. A similar mechanism of uplift has been postulated for the Northern Pennines (Bott & Masson-Smith 1957, Bott 1967) and for the Dartmoor region (Bott et al. 1958). This type of vertical movement differs from the conventional types of isostasy in that the compensating mass deficiency is in the upper crust. The width of the regions affected is somewhat smaller than is usually connected with isostasy. The uplift probably takes place in response to faulting in the upper crust.

4. Vale of Eden

(A) GRANITE RIDGE AT DEPTH JOINING SI-IAP AND WEARDALE GRANITES A belt of relatively low gravity anomalies crosses the southern part of the Vale of Eden in a SW-NE direction,joining the Shap gravity low to the Alston Block low attributed to the Weardale granite (Bott & Masson-Smith I957, Bott 1967). One possible explanation is that a thick sequence of Lower Carboniferous rocks underlies the gravity low or part of it. Another possibility is that a deep-seated granite ridge connects the Shap and Weardale granites at depth. The known variations in Lower Carboniferous thickness suggest that both sources contribute to the belt of low gravity anomalies, as indicated below. Between Shap and Appleby, the Lower Carboniferous succession dipping towards the Vale of Eden consists of about I6O m of the Orton Group (C~S) overlain by 25 ° m of Lower and Middle Limestone Groups (Garwood I912 , Rowley i969). The Limestone Groups are almost identical in thickness and succession to the Alston Block sequence. Thus the Shap-Appleby succession is of thin block type in structural continuity with the Alston Block sequence, both regions slowly subsiding together during the Visdan. In contrast, in the Raven- stonedale-Kirkby Stephen districtthe Lower Carboniferous succession, comprising about 600 m of pre-D zone beds and 60o m of the D I and D2 zone Limestone Groups, is believed to form the west end of the Stainmore trough of thick Lower Carboniferous (Turner 1927, Bott I967). The hinge belt between the thin block succession to the north and the trough follows the Lunedale and Swindale Beck faults between the Alston Block and Stainmore. On the west side of the Vale of Eden the succession thins between Ravenstonedale and Shap; in the intervening region the exact line of the hinge is indeterminate. Assuming a density contrast of o. 15 g cm-3 between Lower Carboniferous and Lower Palaeozoic basement, the trough would contribute --7 mgal towards the gravity low. North of the trough, however, the relatively thin Shap-Appleby sequence of Lower Carboniferous rocks would only contribute about -2 mgal assuming there is no unseen thickening of the Orton Group. Thus the northern part of the belt of low gravity must be substantially attributed to a basement

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feature such as a deep granite ridge, and this is supported by obvious extension of the Shap anomaly towards the NNE (Fig. i). Further east the gravity effect of the postulated granite ridge is masked by the anomaly caused by the Permo-Triassic rocks, but minimum Bouguer anomalies occur where the postulated granite ridge underlies the Permo-Triassic trough. Thus the belt of low gravity between the Shap region and the Alston Block is partly attributed to the Stainmore-Ravenstonedale trough of thick Lower Carboniferous rocks, and partly to a deep granite ridge within the Lower Palaeozoic basement which connects the northern unexposed part of the Shap granite to the Weardale granite. The boundary between the two structures cannot be accurately located on present evidence. The Weardale granite is dated at 4io -J- io m.y. (Holland & Lambert I97o ) which does not differ significantly from that of the Shap granite. Thus the Wear- dale granite appears to form the eastern end of a largely unexposed but continuous belt of granite over 120 km long which underlies much of the Lake District and Alston Block, and passes beneath the southern Vale of Eden between. The granite appears to have exerted a stabilizing effect on the Lower Carboniferous subsidence of the Alston Block (Bott 1967) and of the Shap-Appleby district. An east-west belt involving rapid subsidence in the Lower Carboniferous occurs to the south of the granite ridge.

(B) THE PERMO-TRIASSIC TROUGH Low Bouguer anomalies over the Permo-Triassic trough of the Vale of Eden were discovered by White (i 949) on the basis of a few gravity stations. A more detailed survey has now been made (Fig. i). The low gravity is partly caused by low density Permo-Triassic and Carboniferous rocks, but in the southern part of the Vale the postulated granite ridge and Lower Carboniferous trough enhance the anomaly. Profile EE' (Figs. i and 7) has been chosen for interpretation as it is as far as possible removed from these disturbing effects. The regional anomaly in Fig. 7 allows for the Weardale granite anomaly at the east end. The residual anomaly has been interpreted in terms of subsurface distributions of Carboniferous and Permo-Triassic rocks, assuming density values based on observations (Table I). The interpretation must be ambiguous as two to three interfaces are involved; thicknesses of St. Bees Group, Penrith Sandstone and Carboniferous can to some extent be set against each other without spoiling the fit. To indicate the range of possible structure in the vicinity of the Pennine faults, two extreme models have been obtained. Figure 7(a) is based on the minimum possible thickness of Permo-Triassic rocks adjacent to the outer Pennine Fault, and provides an estimate of the maximum likely thickness of Carboniferous here. Figure 7 (b) is based on zero thickness of Carboniferous just west of the Pennine faults and this model provides a maximum estimate for the thickness of the Penrith Sandstone west of the fault. A thickness of about i km of Lower Carboniferous and Namurian rocks is required to fit the gravity profile west of the Permo-Triassic outcrop, which agrees with stratigraphical estimates and confirms that the Carboniferous rocks here are only marginally thicker than on the Alston Block. Figure 7(a) shows that the

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Carboniferous thickness must decrease towards the Pennine faults. This thinning cannot be stratigraphical, and must therefore be caused by tilting and erosion of the Carboniferous prior to deposition of the Penrith Sandstone. The Namurian and possibly younger Carboniferous rocks are thus preserved in a synclinal structure beneath the trough, and are progressively overstepped towards the Pennine fault belt. Figure 7(b) raises the possibility that Lower Palaeozoic rocks directly underlie the Penrith Sandstone west of the faults, and that these are thrust over Carboniferous rocks by an extension of the Inner Pennine fault belt which here lies west of the Outer Pennine fault. Figure 7 shows that the base of the Permo-Triassic rocks is deepest beneath the centre of the trough, and not adjacent to the Pennine faults where it is quite shallow, contrary to normal geological expectations. Assuming that the Carbonif- erous rocks do not thicken beyond i km beneath the trough, Fig. 7 shows that the Permo-Triassic rocks reach a maximum thickness of about I km and that the Penrith Sandstone is about 900 m thick beneath the centre of the trough. This is substantially thicker than the estimate of 460 m based on stratigraphical evidence (Taylor et al. 1971). The discrepancy between the estimates from surface geology and gravimetry can be reconciled if Penrith Sandstone thickens stratigraphically from the sides towards the centre of the present trough. The overall picture emerging is that the present Permo-Triassic trough originated by contemporaneous subsidence. A discrepancy between stratigraphical and gravitational estimates of thickness of New Red Sandstone rocks occurs in some other regions of Britain, notably the

mgal REGION~

E (WSW) .,2.23 (ENE} E t

km s s ~ ,. s s 5 s $ i

toga I "2 F _ i. - - " - " ..... =, - - • _ residuals - 2 =,,,,2.23 (b) O~ s~ ~.' ...... ^'43 ~ ,': ..... ~ - ~t-~ 2-75~` ~,-~~ ~ ~" ~"" "~~'~..'2.7.~

2 t- 5 ~ ~ ~ s ~ s s s s ~ s & s~ • s s s s s

,'o' -'20 km F I o. 7- Two-dimensional interpretation of the Bouguer anomaly across the Vale of Eden along profile EE' of Fig. I, showing two alternative models. Density values have been assumed as follows: St. Bees Beds--2.23 g cm-s, Penrith Sandstone-- 2"43, Carboniferous rocks--2.63, and Lower Palaeozoic basement--2.75 g cm -s.

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Worcester basin (Cook & Thirlaway 1955) , the Dumfries and Lochmaben basins (Bott & Masson-Smith I96O), and the Stranraer basin (Mansfield & Kennett i963). In each region, the gravimeter records substantially greater thicknesses than are indicated by geological mapping, suggesting contemporaneous subsidence affecting the region. The hinge belts across which the differential subsidence occurs are typically orientated NW-SE, or north to south, in contrast to the dominant east to west hinge belts of Carboniferous age. The Vale of Eden Permo- Triassic trough appears to conform to this tectonic pattern observed elsewhere in Britain.

(C) STRUCTURAL HISTORY OF THE PENNINE FAULT BELT During the Carboniferous, the Alston Block and the Vale of Eden north of Appleby were probably in structural continuity, stabilized by the underlying granite ridge. Similarly, the southern part of the Vale was in structural continuity with Stainmore as a subsiding trough. The Pennine fault system was initiated during the Hercynian movements along what may have been an earlier line of weakness bordering the Weardale granite (Bott 1967). Hercynian faulting (Inner Pennine faults) and thrusting produced a downthrow to the east. Later Tertiary normal faulting caused downthrow to the west on the Outer and Middle (?) Pennine faults. Interpretation of the effects of the Hercynian movements on the Alston Block and the Vale of Eden has in the past led to a controversy which this new interpretation may resolve. Turner (1927) and Trotter & Hollingworth (1928) considered that the Alston Block was depressed and the Vale of Eden was uplifted with the Lake District as a result of the Hercynian Pennine faulting. This interpretation conflicts with evidence on the provenance of rock fragments found in the Brockrams of the Penrith Sandstone. Most of the pebbles are probably derived from uncovering of the Lake District (Turner 1927). However, two quartz dolerite pebbles found in the Upper Brockram of George Gill have been con- vincingly identified as Whin Sill or dyke material by Dunham (1932). It is known that the Whin Sill or dykes do not penetrate significantly west of the Pennine faults (Wadge et al. 1972) and that comparable dolerites do not occur in the Lake District or the Howgill Fells. Thus the quartz dolerite pebbles indicate some contemporaneous erosion of pre-Permian terrain to the east of the Vale of Eden. The structural history of the Pennine fault line and the Vale of Eden can be re-assessed, taking the new gravity evidence into account, in terms of four stages as follows: Stage z (Fig. 8(a)): Hercynian east-west compression produced reverse and thrust faults along the line of the Inner Pennine fault(s) and a complementary syncline along the line of the present Vale of Eden. This resulted in a youthful topography, with a topographic ridge formed along the upthrow side of the fault and a parallel topographic depression along the Vale of Eden. The Alston Block was probably not depressed. During or shortly after these movements, the Whin Sill was intruded into the Lower Carboniferous rocks of the Alston Block; its passage west

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of the fault belt was inhibited by the structure across the fault belt (Trotter & Hollingworth 1928 ) and by the increased pressure beneath the topographic ridge. Stage 2 (Fig. 8(b))" After the Hercynian movements erosion widened the youthful Vale of Eden, removing the topographic ridge along the Pennine fault line and possibly forming an early Pennine escarpment with

/ //I WHIN SILL

3 "~a5 Y s 5 ~ ~ s 3 s a $ $ i - X LOWER PALAEOZOIC S $ S$ 5 5 'aS '

Thrust-faulted monoclinal fold and complementary synclinal sag produced by Hercynian movements, with contemporaneous erosion of newly formed thrust ridge.

3 S .~ 5":' 5 $ • .~ .S $ 3 o. .f 5 . s s .~ ' 15 a 3 "~ 5 $ 3 3 • Permo-Triassic subsidence of the Vale of Eden with contemporaneous erosion of the sides of the valley and depositions of the dominantly elastic succession.

(C} "-- ...... _...... ~ ~y. _-~= ~so~o,c

o S j .1 ~ ~ ---" $ t 51 $ ,$ "J & "_ J $ $ 3 $ ~ i

After completion of Permo-Triassic subsidence and sedimentation, later Mesozoic sediments may possibly cover the region.

- ~ ~ ~ - ~ .~..~-~ ~--~..-/..-".6" .s ~/~:~- s ".s $ " -J ,I "-- ~ ~ s J $ I" J-f J S ,S ,s j 3 J 5 ~ ;,$ $1 •.~ .~ S.~ S$..I,S S.~ $ s°$/ I.

SW 0 10 20 km NE I l I Tertiary normal faulting along present Pennlne fault line followed by erosion. FIo. 8. Sketch sections (vertical exaggeration ×2) showing a new interpretation of the structural history of the Pennine fault line and the Vale of Eden in four stages.

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exposure of Whin Sill or dykes. The basal Permo-Triassic deposits would thus overstep the Carboniferous rocks forming the NE limb of the Vale of Eden syncline onto Lower Palaeozoic rocks adjacent to the Inner Pennine fault. Stage 3 (Fig. 8(c)) : Contemporaneous subsidence affected the Vale of Eden during Permo-Triassic sedimentation. The subsidence was controlled by basement faulting parallel to the Pennine fault line, and may have occurred as a result of east-west tension related to the initial stage of opening of the North Atlantic (formation of Rockall Trough). Differential subsidence ceased at the end of the Permo-Trias, but later Mesozoic rocks may have been deposited across the region and removed by subsequent erosion. Stage 4 (Fig. 8(d)): Tertiary normal faulting (Outer Pennine fault) with downthrow to the west contemporaneous with uplift of the Pennines. The Cross Fell inlier occurs where this fault lies to the west of the Inner Pennine fault(s); further northwest along the Pennine fault line, the Tertiary normal fault lies to the east of the Inner Pennine fault(s) producing the type of structure shown in Figs. 7 and 8 with the Hercynian faults and Lower Palaeozoic belt transgressed by the Permo-Triassic rocks. The throw of the Outer Pennine fault is probably about 3oo m. This new interpretation of the structural history of the Pennine fault line accounts for the presence of Whin Sill or dyke pebbles in the Upper Brockram without raising structural problems. The apparent throw of the Tertiary Outer Pennine fault appears to be much less than has been previously supposed, as a result of structural thinning of the Carboniferous and stratigraphical thinning of the Permo-Triassic rocks northeastwards towards the fault belt. The northeast- ward dip of the Carboniferous and Permo-Triassic rocks off the Lake District towards the Pennine fault line is thus mainly attributed to the Hercynian and Permo-Triassic movements rather than to the Tertiary movements.

ACm'~OWLEDOEIVmNTS. Mr G. Dresser, Mrs Lucy Mines, Mrs Hilda Winn and Mrs Margaret Watson are thanked for technical assistance, Mr W. B. Harland and Dr D. Masson-Smith for their contributions to density determinadons, and Dr G. A. L. Johnson for critically reading the manuscript. Financial assistance from the Royal Society is gratefully acknowledged.

5. References

BOTT, M. H. P. I964. Gravity measurements in the north-eastern part of the Irish Sea. Q. Jl. geol. Soc. Lond. •2o, 369-396. x967 . Geophysical investigations of the northern Pennine basement rocks. Proc. Yorks. geol. Soc. 36, i39-i68. , DAY, A. A. & M.ASSON-SVaTH, D. I958. The geological interpretation of gravity and magnetic surveys in Devon and Cornwall. Phil. Trans. R. Soc. 251A, x6I-I9I. & M~SSON-SmTH, D. I957. The geological interpretation of a gravity survey of the Alston Block and the Durham Coalfield. Q. Jl. geol. Soc. Lond. ix3, 93-I x7. & MASSoN-SMITH, D. I960. A gravity survey of the Criffell granodiorite and the New Red Sandstone deposits near Dumfries. Proc. Yorks. geol. Soc. 32, 317-332.

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& TUSON, J. I974. Interpretation of gravity surveys over the Tertiary volcanic centres of Skye, Mull and Ardnamurchan. Rep. Inst. geol. Sd. (in press). BRowN, P. E., MILLER, J. A. & SOP~R, N. J. i964 . Age of the principal intrusions of the Lake District. Proc. Yorks. geol. Soe. 34, 331-342. COOK, A. H. & THIRLAWAY, H. I. S. I955. The geological results of measurements of gravity in the Welsh Borders. Q. Jl. geol. Soc. Lond. x Tx, 47-7o. DUNHAM, K. C. 1932. Q uartz-dolerite pebbles (Whin Sill type) in the Upper Brockram. Geol. Mag. 69, 425-427 . DWERRYHOUS~, A. R. 19o 9. On some intrusive rocks in the neighbourhood of Eskdale (Cumber- land). Q. all. geol. Soc. Lond. 65, 55-8o. EASTWOOD, T., DIXON, E. E. L., HOLLINGWORTH, S. E. & SMITH, B. 193I. The geology of the Whitehaven and Workington district. Mere. geol. Surv. U.K., xi + 3o4 pp. H.M.S.O. London. EASa'WOOD, T., HOLLINGWORTH, S. E., ROSF, W. C. C. & TROTTFR, F. M. 1968. Geology of the country around Cockermouth and Caldbeck. Mere. geol. Surv. U.K., x + 298 pp. H.M.S.O. London. FOLK~tA-~N, Y. I969. Gravity investigations in the Egremont area, west Cumberland. M.Sc. dissertation, University of Durham. GARWOOD, E. J. x912. The Lower Carboniferous succession in the north-west of England. Q. Jl. geol. Soc. Lond. 68, 449--586. GRANTHAM, D. R. I928. The petrology of the Shap granite. Proc. Geol. Ass. 39, 299-33 i. GREEN, J. F. N. 1917. The age of the chief intrusions of the Lake District. Proc. Geol. Ass. 08, i-3 o. HARKER, A. & MARR, J. E. x89 i. The Shap granite and the associated igneous and metamorphic rocks. Q. dl. geol. Soc. Lond. 47, 266-328. t'In'CHEN, C. S. t934. The Skiddaw granite and its residual products. Q. Jl. geol. Soc. Lond. 9o, 158-2oo. HOLLAND, J. G. & LAMB~.Ra', R. ST. J. i97o. Weardale granite. In Geology of Durham County (Eds G. A. L. Johnson and G. Hickling). Trans. nat. Hist. Soc. Northurab. 4 x, IO3-118. HOLLINGWORTH, S. E. I954. The geology of the Lake District--a review. Proc. Geol. Ass. 65, 385-402. JACKSON, D. E. I96I. The stratigraphy of the Skiddaw Group between Buttermere and Mungris- dale, Cumberland. Geol. Mag. 98, 515-528. M~SFX~.LD, J. & I~Nm~Ta', P. I963. A gravity survey of the Stranraer sedimentary basin. Proc. Yorks. geol. Soc. 34, I39-I5I. M.mu~, J. E. x916. Geology of the Lake District. xii + 220 pp. Cambridge University Press. M~SoN-SMITH, D. 1958. The density and allied properties of rocks. Ph.D. thesis, University of Cambridge. MILLER, J. A. I962. The potassium-argon ages of the Sldddaw and Eskdale granites. Geophys. J.R. astr. Soc. 6, 391-393 . Mn'CH~LL, G. H. x956. The geological history of the Lake District. Proc. Yorks. geol. Soc. 3 o, 4o7-463 • Mos~L~Y, F. i972. A tectonic history of northwest England. Jl. geol. Soc. Lond. x28, 561-598. I~STALL, R. H. x9o6. The Buttermere and Ennerdale granophyre. Q. Jl. geol. Soc. Lond. 6% 253- 274. I9XO. The Sldddaw granite and its metamorphism. Q. Jl. geol. Soc. Lond. 66, I I6-I41. RosE, W. C. C. 1954. The sequence and structure of the Skiddaw Slates in the Keswick-Buttermere area. Proc. Geol. Ass. 65, 4o3-4o6. ROWL~Y, C. R. I969 . The stratigraphy of the Carboniferous Middle Limestone Group of west Edenside, Westmorland. Proc. Yorks. geol. Soc. 37, 329-35 °. SHOTTON, F. W. I935- The stratigraphy and tectonics of the Cross Fell inlier. Q. Jl. geol. Soc. Lond. 9 x, 639-7o4. SImpSON, B. 1934. The petrology of the Eskdale (Cumberland) granite. Proc. Geol. Ass. 45, x7-34. TAYLOR, B. J., BURGESS,I. C., LAND, D. H., MILLS, D. A. C., SMITH,D. B. & WARREN, P. T. 197I. Northern England (fourth edition), British regional geology. 12i pp. H.M.S.O. London. TROTTER, F. M. & HOLLINGWORa'H, S. E. I928. The Alston Block. Geol. Mag. 65, 433-448. 3

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TROTTER, F. M., HOLLINGWORTH, S. E., EASTWOOD, T. & RosE, W. C. C. t937. Gosforth district. Mem. geol. Surv. U.K., vii + I36 pp. H.M.S.O. London. TURNER, J. S. x927. The Lower Carboniferous succession in the Westmorland Pennines and the relations of the Pennine and Dent faults. Proc. Geol. Ass. 38, 339-374. WADGE, A. J., HARRISON, R. K. • SNELLING, N. J. i972. Olivine-dolerite intrusions near Melmerby, Cumberland, and their age-determination by the potassium-argon method. Pro¢. Yorks. geol. So¢. 39, 59-70. WHITE, P. H. N. i949. Gravity data obtained in Great Britain by the Anglo-American Oil Company Limited. Q. Jl. geol. Soc. Lond. xo4 (for I948), 339-364.

Received 3 September I973; revised typescript received 13 October I973; read 9 January I974.

Professor M. H. P. Bott, Department of Geological Sciences, University, South Road, Durham. DISCUSSION SIR KINGSLEY DUNHAM: Professor Bott's gravity study has contributed materially to the understanding of NW England. The light it throws on the nature of the Pennine fault-system shows that the importance of the compressional inner faults, first brought out by F. W. Shotton, is even greater than was supposed. It is gratifying to have a model which is able to accommodate the Whin Sill and thus to explain the pebbles in the Lower Brockram, the first of which was found by Arthur Holmes. He wished to ask Professor Bott three questions: (I) how significant is the position of the base of the granite batholith, shown on the model as at about 9 km depth over the larger part of the Lake District, but at 6 km plus at Skiddaw, and would greater or somewhat lesser depths invalidate the model ? (2) Does the survey throw any further light on the sub-Carboniferous structure of S Cumberland and Furness ? (3) Does the postulated link between the Lake District and Weardale batholiths pass beneath the exposed part of the Cross Fell Inlier ?

MR I. C. BURGESS asked Professor Bott if the "granite ridge" anomaly in the Vale of Eden could be accounted for by the presence of very thick Carboniferous Basement Beds ? He then commented: Much new stratigraphical and structural information on the development of the Pennine Line was obtained during the resurvey of the Cross Fell area by the Institute of Geological Sciences; this is summarized in the guide (Burgess, I. C. & Wadge, A. J., in press. The Geology of the Cross Fell area) to accompany the recently published I :25,ooo Special Sheet of the Cross Fell Inlier. The existence of a westwards-facing escarpment along the southern part of the Pennine Line during the Permian is substantiated. The Penrith Sandstone 'Upper Brockram' shows evidence of eastwards deri- vation. The clasts include sporadic Whin Sill pebbles, but, much more commonly, Carboniferous limestone and sandstone, notably the easily identifiable Roman Fell Sandstone; and in places they show imbrication consistent with deposition by westwards flowing streams. Also, the 'Upper Brockram' appears to die out

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westwards. About i oo m of almost exclusively water-laid material are seen in Hilton Beck, whereas in George Gill i. 5 km to the west, the corresponding strata contain only wedges of brockram in a dune-bedded sandstone sequence. A remnant of the Permian escarpment is preserved on Roman Fell (west of Professor Bott's Stage i thrust line). The strata on the face of the escarpment, both the Lower Palaeozoic rocks and the overlying Roman Fell sandstones, are deeply red-stained. This reddening dies out eastwards, away from the scarp face, and the rocks on the face of the fell provide a possible source for many of the pebbles in the brockram. The Penrith Sandstone just west of the 'Outer Pennine Fault' on Roman Fell rests unconformably on reddened Carboniferous (probably Namurian) sandstones and siltstones. Their preservation in this situation, at a lower topographic level than the Roman Fell Sandstones just east of the fault, implies a down- west movement, possibly as much as 8oo m, along the 'Outer Pennine Fault' in the early Permian (with subsequent down-west throw of 3oo m in Tertiary times). The northwards continuation of this structure may be identified east of Milburn, where reddened Lower Carboniferous rocks are faulted against reddened rhyo- litic welded tufts. The throw of the fault cannot be determined here, but must be of the same order of magnitude. This interpretation bears a close resemblance to that indicated for the Penrith area in Professor Bott's Fig. 7 b, but modified by the presence of a down-west fault bounding the Carboniferous rocks on the east. His suggestions on geophysical grounds of eastwards thinning of both Carboniferous rocks and Penrith Sandstone, the former by overstep, the latter by overlap, are both consistent with the stratigraphical evidence in the southern part of the Vale of Eden.

DR C. D. V. WILSON asked if Professor Bott could comment on the magnetic fields of the granites. If the density of the Eskdale and Shap granites increased towards their margins, as shown in one of his models, this implied change of composition might be accompanied by a change in magnetic properties. If so, this might help in discriminating between this model and that of a uniform granite thinning towards the edge.

DR D. H. MATTHEWS said that in I97O and I97I two gravity traverses were made across the Vale of Eden and the northern part of the Cross Fell inlier through the villages of Melmerby and Knock to investigate the nature of the outer Pennine Fault and the depth of the New Red Sandstone basin. The details of the profiles and the interpretations placed upon them agree very well with those of Professor Bott who has interpreted a section several miles further north than ours, clear to the north of complications introduced by the inlier and the Shap-Weardale granite connection. Several refraction lines were also shot: a first interpretation of one reversed line along the Eden River east of Penrith over a range of 6 km, obtained a southward dipping refractor with a velocity of 5'8 km sec -1 at a depth of I km which could be anhydrite (Vp 5"9 km sec-1), Carboniferous limestone (6-o km see -1) or Borrowdale Volcanics (5"5 km see-l).

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It would aid the interpretation of the gravity on our traverses if Professor Bott could put some limits on the gravity effect of the granite that he suggests connects the Shap and Weardale granites and contributes to the gravity low in the Vale of Eden east of Penrith across which our lines were run.

PROFESSOR R. M. SHACKLETON: One of the interesting results of this gravity survey is to show that the level to which the granites rose was rather uniform over quite a wide area. This had also been demonstrated elsewhere. Yet in the area investigated the rocks forming the roof of the granite were apparently still denser than the granite magmas. What stopped the granites from rising further ?

DR E. J. w. JONES: On a recent gravity map of the Irish Sea (M. Bacon & R. McQuillin; Jl. geol. Soc. Lond. i972 , I28, p. 614), the centres of four roughly circular Bouguer anomalies fall on the southwestern projection of the axis of the Lake District batholith. One of these anomalies lies just seaward of the Eskdale granite, while two of the others appear to be associated with sedimentary basins. Does Professor Bott believe that this line of anomalies is related to the extension of the Lake District batholith at depth beneath the Irish Sea, or does he think that the batholith terminates close to the coast ?

THE AUTHOR thanked Sir Kingsley Dunham for his kind remarks and made the following points in reply: (I) The depth to the base of the granite density contrast was determined using surface density values, and if the true densities vary with depth then the estimated depths will be somewhat in error. One can be fairly confident that the true depth lies between about 6 and I8 km. (8) The gravity results in South Cumberland and Furness are in good agreement with the known geological structure, with the Permo-Triassic and Carboniferous successions thickening into the East Irish Sea basin as indicated in Fig. 4 of the earlier Irish Sea gravity paper (Bott 1964). (3) The relatively low Bouguer values of about --2 mgal over the central and southern parts of the Cross Fell inlier indicate that the postulated link between the Lake District and Weardale batholiths does underlie the Cross Fell inlier at depth.

In reply to Mr Burgess, a thickness of at least I "5 km of Carboniferous basement beds would be needed to account for the eastward extension of the Shap negative anomaly (assuming --o.i5gcm -3 Carboniferous/Lower Palaeozoic density contrast). The continuity of the belt of steep gradients north of Shap with those over the Carboniferous region to the east, the occurrence of low gravity values over the Cross Fell inlier (see above), and the Carboniferous stratigraphy of the Shap-Appleby region, all support the granite ridge hypothesis; however, some contribution to the gavity low from thick Carboniferous basement beds cannot be ruled out. The new geological evidence on the history of the Pennine line presented by Mr Burgess is of great interest and appears to be in excellent agreement with the conclusions from the gravity interpretation.

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In reply to Dr. Wilson, the I.G.S. aeromagnetic map shows a relatively feature- less magnetic field over much of the Lake District, with more disturbed areas overlying the environs of the Ennerdale granophyre, the southern margin of the Shap granite, and the northern belt of Borrowdale Volcanics. The magnetic anomalies do not appear to be sufficiently definitive to add to the sub-surface interpretation of the granites, although some ground observations may be worth making with this in view.

Professor Bott was interested to hear of the new refraction result given by Dr Matthews. The contribution to the minimum Bouguer anomalies over the Permo-Triassic trough ESE of Penrith arising from the postulated granite ridge beneath is probably between --7 and --15 mgal, but it cannot be pinned down accurately because of the ambiguity problem. The new refraction result, possibly supplemented by further well-placed refraction lines, may help to clarify the interpretation.

In reply to Professor Shackleton, the author is not aware of a convincing answer to this problem or to that of why batholiths rarely, if ever, break the surface. A possible suggestion is that the upper boundary of the batholith cools more rapidly as it becomes nearer to the surface so that the increasing stiffness of the magma progressively discourages stoping.

In reply to Dr Jones, the East Irish Sea gravity low between the Isle of Man and SW Cumberland is almost certainly caused by thick sediments as indicated by gravity interpretation (Bott i964; also M. H. P. Bott & D. G. G. Young, Q. J1. geol. Soc. Lond. I97I , I26, pp. 413-434) and by the seismic refraction results of Bacon and McQuillin. On land, the known substantial thickness of Permo- Triassic strata in the vicinity of Seascale indicates that the gravity anomaly caused by the granite is closing seawards, and that the batholith probably terminates before the coast is reached. The two gravity anomalies of different origin merge into each other near the coast. Similar relationships are observed between the Kish Bank sedimentary basin and the Leinster granite (Bott & Young I97 I), and between the Dumfries basin and the Criffell granodiorite (Bott & Masson- Smith 196o) ; these subsidences may possibly be encouraged by contemporaneous uplift of a "yoked" granite.

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