GEOLOGICAL SURVEYING 313 
acted upon by the atmosphere and surface water. The 
stratified deposits in question have often yielded relics of an 
old land-surface, and are thus evidence that the formation of 
boulder-clay was an interrupted and not a continuous process. 
The same inference may be drawn from the occurrence of 
marine fossiliferous deposits included in till. 
The relation of the boulder-clay to the immediately sub- 
jacent rocks is deserving of study. The latter are sometimes 
so broken, jumbled, and confused, that it is hard to say where 
the’ shattered and disturbed rock ends and_ boulder-clay 
begins. The student should note whether the slabs and reefs 
of rock have been bent over or wedged out of their beds, and 
the direction in which they have moved should be ascertained. 
Instead of being broken and jumbled, the subjacent rocks 
may show a smoothed, polished, and striated surface. The 
compass bearing of the strize should always be taken, as this 
indicates precisely the direction of ice-flow at the point of 
observation. It is possible that the beginner may at first 
have some difficulty in distinguishing between a glacially 
striated surface (Plate LII.) and slickensides (Plate XXXVI). 
The latter, however, are usually confined to flat or even 
surfaces, and are frequently glazed with mineral matter—the 
scratches, moreover, are strictly parallel. It will be noted 
further that when a slickensided surface shows any depres- 
sions these are not striated. Glacial striz, on the other hand, 
may occur on flat, convex, concave, or rapidly undulating 
surfaces. The smoothing and polishing is not confined to 
the protuberances upon a rock-surface, but every little dimple 
and depression is equally dressed. Although roughly parallel, 
glacial striz are yet not so straight as slickensides, but often 
cross each other at acute angles. Frequently, indeed, they 
may be seen curving gently round the sides of projecting 
knobs, as if these had caused some slight deviation of the ice- 
flow. The scratches may be as fine as if drawn by an 
engraver’s needle, or they may be coarse, jagged ruts; and 
between these extremes all gradations occur, and may be seen 
side by side on the same rock-face. 
Roches moutonnées.—Dressed rock-surfaces occur not only 
underneath boulder-clay, but on exposed hill-slopes and rocky 
elevations, from which the boulder-clay has been stripped by 

