, 8 The Irish Naturalist. January > 



As previously mentioned, Boulder-clay formerly occupied a 

 much larger area than it appears to do now, this being indi- 

 cated by the presence of glacial erratics of Old Red Sandstone 

 at elevations well above the 250 ft. contour The presence of 

 a good deal of limestone in the drift is indicated also by the 

 deposit of calcium carbonate formed on the shore at Fresh- 

 water Bay by the small streamlet draining the principal valley 

 north of the central rock ridge of the island. 



Local drift occurs in some of the minor hollows between 

 rock slopes and in other places favourable for the accumula- 

 tion of the detritus derived from the gradual decay of the 

 local rocks. These areas are shown on the accompanying map 

 the same colour as the boulder-clay deposits, but cross-hatched 

 with broken lines running N.W. and S.E. Though an 

 occasional round pebble may be found in this deposit, as a 

 rule it consists exclusively of local material, the fragments 

 being in an angular condition. The largest area of local 

 detritus occurs to the S.W. of Trinity Well, and is composed 

 of material washed down through the rock-gorge near this 

 point, and deposited as a kind of delta at a lower level. 

 A few rounded pebbles derived from the boulder-clay occur in 

 this deposit, which mainly consists of andesitic debris. 

 Besides the areas of local detritus shown on the map, a very 

 thin skin of identical material occurs over much of the ground 

 shown as bare rock ; in a few places this is sufficiently thick 

 to support a scanty herbage, but was considered too thin to 

 be mapped with the local drift deposits, which are, relatively 

 speaking, considerably thicker, and may be regarded as of 

 economic value. 



Raised beach. — Indicative of a comparatively recent upward 

 movement of the land, is the raised beach deposit fringing in 

 a narrow strip the western side of the island a little north and 

 south of the harbour. At the latter place it attains its 

 maximum development, and consists of the usual blown sand, 

 beach shingle, and shells, and traces of midden deposits. 

 Towards the south it merges gradually into a pebbly storm 

 beach which generally shows a terraced cross section. 

 Amongst interesting erratics found in this deposit are the 

 Ailsa Craig granophyre, and a granite like that from the 

 Mourne mountains, Co. Down. A fair number of flints also 



