164 E. C. Martin — Source of Limesione Pebbles 



Source of the Limestone Pelhles. 



Althougli the fossils in the limestone pebbles do not appear to admit 

 of exact determination, the evidence seems quite sufficient to justify 

 the conclusion that they have been derived from Carboniferous Lime- 

 stone of the Mendip or Avonian type. A few of the normal Carboni- 

 ferous fossils (including Spirifers and a doubtful Zaphrcntis) occur in 

 the Culm limestones, but they appear to be rare exceptions/ whilst 

 the usual fossils are Posidonomyse and Goniatites, which, in the 

 neighbourhood of Burlescombe, are much more common in the shales 

 than in the limestone. But, on the whole, fossils, with the exception 

 of crinoid fragments, are very rare in the Culm limestones ; on the 

 other hand, there is abundant evidence that some, at least, of the 

 limestone in the conglomerate has been derived from a very fossili- 

 ferous rock. The difficulty of getting good specimens of the fossils is 

 due, not to their scarcity, but to the hardness of the stone and the 

 coating of oxide of iron which so often covers the pebbles. 



The Carboniferous Limestone of South "Wales contains fossils similar 

 to those found in the conglomerate, and often silicified ; moreover, 

 a large area of this limestone was exposed to the north and north-east 

 whilst the conglomerate was being formed. It is from this area that 

 some, at least, of the limestone pebbles found in the conglomerate 

 appear to have been derived. 



The origin of the Bunter Pebble - beds of Budleigh Salterton 

 and the Midland Counties has been discussed in numerous papers 

 in the Geological Magazine and elsewhere,^ and the view that 

 these deposits are of fluviatile origin now appears to meet with very 

 general acceptance. In the case of the Budleigh Salterton Bed the 

 ' foreign ' quartzite pebbles have been traced to the old Armorican 

 land lying to the south or south-west. To the north of Budleigh 

 Salterton the pebbles decrease in size, and Mr. H. H. Thomas has 

 found that the percentage of heavy minerals in the matrix of the 

 Pebble-bed falls from 75 per cent, at Budleigh Salterton to 71 per 

 cent, at Uffculme. There is thus good evidence for the theory that 

 the Budleigh Salterton Bed was formed by a large river from the 

 south or south-west. In the neighbourhood of Burlescombe this 

 river appears to have received an important tributary from the north, 

 flowing down the vallej^ between the Quantock and the Brendon 

 Hills and bringing with it pebbles of Carboniferous Limestone. 

 The united stream probably found an outlet to the east. 



On the accompanying sketch-maj) are shown the probable directions 

 of the sediment-bearing currents in East Devon and "West Somerset 

 during the Bunter period. The river from the north probably 

 received tributaries from the Devonian areas on both sides of its 

 valley ; to these streams, which would have had a short but rapid 

 course, are to be attributed the rubbly gravels of little-worn Devonian 

 grit fragments, found between Yellow and Combe Plorey, and the 



^ Rev. W. Downes, B.A., F.G.S., Transactions of the Devonshire Association, 

 1878-9. 



2 A useful summary and discussion of these papers is given by 0. A. Shrubsole, 

 P.G.S., in a paper " On the Probable Source of some of the Pebbles in the Triassic 

 Pebble-beds of South Devon and of the Midland Counties" : Q.J.G.S., 1903, p. 311. 



