148 
The Work of the Geological Survey. 
and the position of the later valley-glaciers. The various boulder- 
clays have been mapped, likewise the sands and gravels, the esker- 
drifts, the marine shelly-clays, and the distribution of erratic blocks. 
A vast amount of information has thus been collected regarding the 
history of the Ice Age in most parts of the country. Even in the 
southern or non-glaciated fringe which I have already referred to 
one of the members of the staff has been able to detect interesting 
evidence that though beyond the limits of the northern ice-sheets, 
this southern tract nevertheless had its frozen soil and its rafts of 
coast-ice. In the north of Scotland proofs have been obtained of 
the long-lingering of the ice-fields in that region ; while in all the 
mountainous districts the gradual retreat of the valley-glaciers, as 
the climate grew milder, has been shown by mapping the successive 
crescents of moraines, one behind the other, up to the very base of 
the crags from the material of which they were formed. 
The survey of the superficial deposits thus combines a wealth of 
geological interest with a great deal of practical value. The geolo- 
gist may find in it the solution of some problems and the presenta- 
tion of many more, whilst the farmer, the water-engineer, the 
builder, and the sanitary inspector may each in turn gain some 
practical information from it for their guidance. 
( b ) Solid Geology Survey . — By way of distinction the mapping 
of the formations of every age that lie beneath the recent superficial 
deposits is known as the survey of the “ solid geology.” The object 
in this part of the work is to represent on the maps the exact area 
which every formation or group of rocks occupies at the surface, 
together with all indications that can be obtained of its structure, 
such as its variations of inclination, its changes of lithological cha- 
racter, and the dislocations by which its outcrop is affected. While 
the basis of this work is rigorously geological, an effort is made to 
ascertain and record any facts which may have an industrial bearing, 
such as the presence of useful minerals, or the depth and variations 
in thickness of water-bearing strata. The large scale on which the 
Survey is conducted allows much local detail, both of a scientific and 
a practical nature, to be inserted on the maps. 
In those districts of the country where the rocks have long been 
well-known and where the geological structure is simple the duties 
of the surveyor are comparatively light, though it often happens in 
these tracts that the simplicity of the solid geology is compensated 
for by a great complexity in the overlying “ drifts.” Yet even 
among formations that have long been familiar the diligent surveyor 
may generally glean new facts or be able to throw new light on 
facts which were already well-known. Thus only a few years ago, 
even in a formation so well worked out as the Chalk, one of the 
members of the Survey detected the existence of a phosphatic 
deposit like those which have long been worked in the Chalk of 
Belgium and France. 
It is where the rocks are varied in character and complicated in 
structure that the full working power of the Survey is called out. 
