GEOLOGY 



with stones in the upper 12 feet, and driftwood at 40 feet from the 

 surface, was penetrated and found to rest on a gravelly beach similar 

 to that at Fremington and at the same level. He also records a well 

 sinking at Roundswell hamlet mill in 1862, through 40 feet of smooth 

 mostly unstratified brown clay to gravel beach. Near Upper Rounds- 

 well the clay is said to be 90 feet thick, thinning eastward and westward. 

 The phenomena suggest an old channel of the Taw filled with gravel 

 contemporary with the raised beaches, subsequently deepened during 

 their elevation and filled with river mud overlain by Head, or subaerial 

 washes from the neighbouring slopes. 



Submerged Rock Valleys, Head, etc. — If the old valley, above described, 

 were re-excavated and submerged it would afford an apposite illustration 

 of the submerged rock valleys of the Dart, Plym, Tamar and Tavy de- 

 scribed by Codrington.* Outside Kingswear jetty a stiff deposit with 

 granite and quartz boulders was proved beneath the silt. The rock bed 

 of the Tavy, in the deepest parts 25 to 68 feet below spring tide low 

 water at the viaduct, was found to be covered by 2 to 4 feet of stiff 

 yellow clay with small granite boulders, under the silt. Patches of clay 

 with stones were found here and there on the rock sides or bottom under 

 the silt which partly fills these submerged valleys. Codrington con- 

 sidered the stony clays to be relics of boulder clay, and that the valleys 

 had been excavated to their present depth before the glacial period. 

 The Yealm and Salcombe creeks may be also regarded as submerged 

 rock valleys. At Puslinch by the Yealm a sinking of 40 feet was made 

 without reaching rock.' 



At Splat Cove a gravel deposit, probably contemporaneous with the 

 raised beaches, occurs, at about 1 5 feet above high water mark, near the 

 mouth of the Salcombe estuary. 



These valleys were no doubt in existence during the raised beach 

 period, but the argument against their excavation to their present 

 depth at that time is rather favoured by the general absence of rock 

 platforms and of contemporary gravels along their borders. The 

 modifying and deepening of existing valleys, especially in their seaward 

 extension, during the period of elevation which succeeded the raised 

 beach formation, accords best with the phenomena described. At Rum- 

 leigh on the Tamar there are terraces marking successive steps in this 

 erosion. The clayey gravel terrace with granite boulders, cited by Cod- 

 rington, bounds the alluvium near Gawton Mine and belongs to the same 

 series of accumulations as the Head on the coast which ' marks a time 

 when the degradation of the surface proceeded much more rapidly 

 than by the mere effects of decomposition, and when fragments of 

 rock far exceeding the motive power of any rainfall were conveyed 

 down slopes along which the minutest particles of matter, only, are 

 now carried." 



* ^ari. Journ. Geol. Soc. vol. liv. pp. 26Z-76. 



* Worth, 'The Geology of Plymouth,' Trans. Plym. Inst. 1875. 



* Godwin-Austin, ' On the Superficial Accumulations of the coasts of the English Channel,' ^art. 

 Journ. Geol. Sec. vol. vii. 



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