
GEOLOGICAL SURVEYING 317 
the deposits consist of evenly bedded gravel, sand, etc., the 
subsequent origin of the mounds and banks will be disclosed 
by the manner in which the horizontal beds are truncated 
by the slopes of the ground. It is more difficult to differ- 
entiate between a true “drum” or “drumlin”—that is, a 
bank or ridge of boulder-clay due to glacial accumulation, 
and banks of the same material which have resulted from the 
irregular erosion of a thick continuous sheet. If the banks 
do not trend in the same direction as the roches moutonnées 
and strie of a district, then they are not true drumlins. 
Should their trend appear to coincide generally with that 
of glaciation, the whole modelling of the surface must be 
studied before we come to the conclusion that the banks 
are original structures. If they have been carved out of a 
sheet of boulder-clay by running water, evidence of this 
should be found in the arrangement of the intervening 
hollows, which will be grouped much in the same way as 
the feeders and tributaries of a stream. In other words, the 
banks and ridges of the district will not be arranged through- 
out in parallel positions, but will fan-out as they are followed 
in a direction opposite to that of the water-flow. Moreover, 
the existing brooks and their feeders, or, should these have 
disappeared, the evidence of their former presence afforded 
by flats and terraces of alluvial deposits occupying the 
hollows, would be sufficient to convince us that the banks 
or ridges were not true drumlins, but secondary structures. 
It is to be noted, however, that true drumlins are of two 
kinds—while some have been accumulated as such under- 
neath the old ice-sheets, others would appear to be merely 
the remnants of widespread sheets of boulder-clay which 
have been exposed to subsequent glacial erosion. In Galloway, 
for example, the low grounds extending outwards from the 
mountains were originally deeply covered with extensive 
sheets of boulder-clay, by the mer de glace that formerly 
overflowed all that region. Long after the disappearance of 
that ice-sheet, great glaciers streamed out from the mountain- 
valleys for some little distance, and trenched and furrowed 
the older boulder-clay—thus forming a series of secondary 
drumlins, 
RAISED BEACHES.—These are flats and terraces occurring 

