TiATERlTlO FOEMATIONSs 53 



South Indian Kailway. A good deal of shingle shows also on the low 



„ , , n v.- ^ i granular quartz ridges east of Timmaraiapuram 

 Patches ot smngle at =» ■<• ° •> ^ 



Timmarajapuram. ^nd thence southward at intervals for a mile or a 



mile and a half south of Vagai Kulam along the road from the rail- 

 way down to the delta of the Tambraparni. Here 

 Soutli of Vagai Kulam. ,.,,,. 



and there these gravels are much mixed with im- 

 pure red hsematitic gravels^ and then get solidified to a great extent. They 

 are everywhere very thinly scattered over the gneiss with too many out- 

 crops of the latter protruding through them to allow of their being 

 separately mapped on a small scale. 



A few extremely small traces of these gravels were met along the 

 Gravel talus of Valla- ©asteru foot of the Vallanad hill^ while a few of the 

 nad hiU. masses of granular quartz forming the rather ex- 



tensive talus on the same side of the hill show distinct traces of round- 

 ing and polishing by water action. There can be no doubt that during 

 the period of depression when the various gravel beds now enumerated 

 were being deposited, Vallanad hill and many of the other hills rising 

 out of the plains of Madura and Tinnevelly districts must have been 

 islands either in the sea or in some very widespread fresh-water lake 

 or lagoon, and that the talus of debris surrounding the various islands 

 must have been exposed to considerable wear by local surf action. Such 

 surf -rolled taluses are well known further north in the northern half of 

 the Carnatic, e.^., the great shingle banks along the south flank of the- 

 Nagari mountain in North Arcot district, and similar banks along the 

 eastern base of the Vellakouda mountains, or Eastern Ghats in Nellore 

 district. The hard and durable nature of the dense varieties of the quartzite 

 has caused the shingle formed from it to maintain its true character des- 

 pite long continued weather action. The brittle character on the contrary 

 of the granular quartz rock will readily account for the degradation of 

 the coarse shingle talus which must have been formed round the islands 

 standing up out of the lateritic sea or lake. Traces of such shingle were 

 noticed also near the base of the Sangani hill (Trigonometrical Station), 

 south of Tinnevelly and on the Pasu Malai south-west of Madura. 



( 53 ) 



