

386 STRUCTURAL AND FIELD GEOLOGY 
Loulder-clay or Till is the general term applied to the 
stony clays. These clays, being of subglacial origin, consist 
almost exclusively of crushed and comminuted rock. In 
other words, they consist of unweathered materials—differing 
in this respect from all clays of truly sedimentary origin. As 
a rule, these stony or boulder-clays are of a highly imperme- 
able character, and consequently, the soil formed upon them 
is usually thin. The subsoil is also thin, and the materials of 
which it is composed show commonly little trace of weathering 
—the most notable change being the partial oxidation of 
ferruginous constituents. ‘Thus, a blue-coloured boulder-clay 
may graduate upwards into a yellowish or brownish clay, two 
or three feet in thickness, overlaid by a few inches of a more 
or less stony, tenaceous clay-soil. It may be said in general 
terms that the colour and composition of boulder-clay are 
determined by the nature of the bed-rock upon which or near 
to which it lies. Thus, in a district where red sandstones 
predominate, the overlying boulder-clay is usualiy reddish 
and more or less arenaceous: where Carboniferous rocks 
prevail (light coloured sandstones, black shales, fireclay, iron- 
stones, coal, limestone, etc.), the till is dull bluish-grey in 
colour, and often exceedingly tenaceous: when the dominant 
country-rock is chalk, the ‘boulder-clay forms a dirty greyish- 
white marl. A good geological map, therefore, although it 
may not show the superficial formations of a country, is 
nevertheless often a reliable index to the average character 
of the boulder-clays. The general local character of the till, 
however, does not hold good throughout. If we follow the 
direction of ice-flow in a country like Scotland, we soon 
discover that there are limits to the local character of the till. 
Coming down the valley of the Tweed, for example, from the 
heart of the Silurian Uplands to the low grounds near 
Melrose, we find that, so long as we are in the region of 
gereywacké and shale, the till is a fawn-coloured, tenaceous, 
eritty clay, crowded with angular and subangular fragments 
of the country-rock. As we leave the Silurian strata and 
enter the region of Old Red Sandstone, the till continues to 
be composed of the débris of greywacké and shale, although 
here and there fragments of the underlying red sandstones 
begin to appear. Continuing down the valley, red sandstone 

