152 EDINBURGH PAST AND PRESENT.
traverses the coal-field near Niddry, while two very conspicuous examples
run across the Carboniferous Limestone series near Prestongrange and Longniddry.
Another great gap, for the filling up of whichno evidence exists in this
part of the country, separates these volcanic rocks from the next geological
events in our chronicle-those of the Ice Age. The neighbourhood of Edinburgh
will always bear a special interest in regard to this part of geology, from
the fact 'that it was here Sir James Hall observed and described, those
dressed ' rock-surfaces which are now everywhere acknowledged to be due to
the grindiog action of ice. They are to be seen on the west slope of Corstorphine
Hill, on the top of the southern part of Salisbury Crags, on the sandstone
at Joppa salt-pans, on the porphyrite at Blackford Quarry, on the top of
Allermuir Hill, one of the Pentlands, at a height of 1617 feet, and in many
other places. The general direction of the strk and groovings is a little to the
north of east, indicating that the mass of ice which produced these markings
moved seawards along the line of the valley of the Firth of Forth.. The roqks
of the neighbourhood of Edinburgh pass beneath masses of glacial drift-the
productof the glaciers, icebergs, and seas of the glacial period. At the bottom
of these deposits lies the boulderclay or till-a stiff dark-blue clay stuck full of
stones of all sizes, up to blocks of a yard in diameter. Many of these are well
smoothed, and striated like the surfaces of the solid rocks underneath. On
examination it is found that the majority of them are of local derivation, that
some have come from distances of ten or fifteen miles to the west, a smaller
proportion from western hills twenty or thirty miles away, while a very small
number have travelled from the Highland mountains. Thus the stones
corroborate the testimony of the striz on the rocks, that the general icemovement
was here from the west. The gradual deflection to east-by-north
,was evidently due to the influence of the shape and direction of the great
valley upon the march of the ice.
While the lower parts of the boulderclay appear to have been formed under
a huge sheet of land-ice moving steadily across the country, like the enormous
icemantle of Greenland, the upper parts of the deposit suggest that they may
have originated to some extent in the sea, either underdw solid ice or under
the broken bergs which flanked the long front of the ice-sheet They contain
larger blocks than the lower parts of the till, and these are often arranged in
rude lines.
The boulderclay, as might be expected, is singularly destitute of fossils.
To the west of Edinburgh,; when the Union Canal was being cut, a well
