ON THE GLACIATJON OF EAST LOTHIAN SOUTH OF THE GARLETON HILLS. IT 



II. Phenomena Illustrating the Oscillatory Nature of the Retreat. 



We have already referred to the great accumulations of fine sands, silts, and gravels 

 which occur in the Upper Keith neighbourhood. They have all the appearance of 

 having been laid down in a great lake, and it seems necessary to suppose that during 

 the time of their formation the ice sheet of the lowlands lay in close proximity to 

 the north and thus served to dam back the waters in which they accumulated. 

 A closer inspection of the many good sections * occurring in this district confirms this 

 interpretation, since such an examination affords independent and clear evidence 

 of the continued proximity of the ice sheet during the formation of the sands. 



Two of the sections, both of them occurring along the course of the Keith water, 

 are specially clear ; the first of them constitutes the Red Scar,+ a little north of Costerton 

 cottage, and the second j lies just to the west of the road from Upper Keith to Ormiston. 

 Their details are as follow : 



Feet. Feet. 



4. Sand . . . 53 Not exposed in this section. 



3. Boulder clay . . 13 Boulder clay (top not seen) . 6 



2. Sand ... 15 Sand 65 



1. Boulder clay (base not Boulder clay (base not seen) 33 



seen) . . . 37 



Even in the Red Scar the section is incomplete, and the upper sands must be fully 

 150 feet in thickness. The thin gravel beds sparingly interbedded with the sands carry 

 a conspicuous proportion of grauwacke pebbles derived from the Silurian Hills two 

 miles to the south, but the larger blocks of the intercalated boulder clay consist of 

 sandstone, etc., so that it seems necessary to suppose that the latter was deposited by 

 the lowland ice sheet. In fact, we take its presence to indicate that this glacier was 

 not melting away as an inert mass of ice, but that it was ready, when climatic conditions 

 favoured, to re-advance on to the floor of the temporary lake which spread out before 

 it, and there deposit a covering of boulder clay upon the sands and silts which had 

 previously been collecting. The intercalated boulder clay can be traced w r ith almost 

 complete certainty for a distance of a mile in a direction parallel with the ice margin, 

 and a quarter of a mile at right angles to the same, without any indication of coming 

 to an original limit, so that the oscillation recorded here seems to have been of con- 

 siderable magnitude. The positions of the best sections in this interesting district are 

 indicated by asterisks placed on the general map. 



Many other exposures occur along the Lammermuirs which tell the same tale as 

 those of Upper Keith, but not in so impressive a manner ; we need therefore only 

 allude here to the readily accessible exposures in the railway cutting soutl of Tynehead 



* The interstratification of sands and boulder clays here was first noticed by Mr Anderson. 

 tPl. IV. fig. 2. 

 J PI. IV. fig. 1. 

 TRANS. ROY. SOC. EDIN., VOL. XLVI. PART I. (NO. 1). 3 



