

310 STRUCTURAL AND FIELD GEOLOGY 
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folded, and assume a basin-shaped arrangement, and when 
such is the case the outcrops are not so hard to trace. 
GLACIAL AND FLUVIO-GLACIAL ACCUMULATIONS.—The 
deposits included under this head are widely distributed in 
this country and in corresponding latitudes of the Continent 
and North America. They cover extensive areas in our 
lowlands—occupying valleys and sweeping over intermediate 
tracts, so as largely to conceal the underlying solid rocks. 
In our mountainous districts they are mostly restricted to 
the valleys, but often extend upwards to considerable heights 
upon the mountains themselves. It would be quite beyond 
the limits of this work to attempt any detailed description 
and classification of these accumulations. Attention is, 
therefore, limited to some of the salient phenomena presented 
by the more notable of the deposits in question. 
(a) Boulder-clay or Till—This is an unstratified or 
amorphous mass, the essential lithological characters of which 
have already been given (see p. 63). One of its most 
striking peculiarities are the stones and boulders which it 
contains. These are almost invariably fresh, unweathered, 
and generally blunted and subangular in shape—often show- 
ing smoothed, polished, and striated faces. The beginner 
should note especially the character of the striation and its 
relation to the shape, size, and species of the stones. Usually, 
stones which are longer than broad are most distinctly 
striated lengthways, while those which are as broad as they 
are long are striated equally in all directions; very large 
blocks are often smoothed on one side only, while smaller 
boulders and stones are commonly smoothed all over; again, 
compact fine-grained rocks, such as limestones, shale, iron- 
stone, felsite, etc., have usually received a better polish than 
coarse-grained grits, sandstones, etc. The observer should 
be on the outlook for any traces of arrangement of the stones 
and boulders. Occasionally, lines of small and large boulders 
may be seen traversing the face of a cutting in boulder-clay— 
the boulders not infrequently lying lengthways. Sometimes 
the upper surfaces of such “boulder-pavements,” as they are 
termed, are distinctly striated in one common direction. The 
student should also subject the gritty clay itself to a close 
examination. A portion should be taken home and 
