342 
which would become lakes or peat-mosses if the till were extracted. 
This distribution of the detritus, Mr. Lyell observes, may be ex- 
plained on the supposition that, if the cold period came on slowly, 
the advance of the glaciers would push forward the detritus accumu- 
lated at their termination, and fill up, wholly or in part, the lakes or 
other cavities which they would encounter in their progress. Along 
most of the river courses, and in the lowest depressions of Strath- 
more, the till is covered by stratified sand and gravel. 
One of the most remarkable peculiarities of the transported ma- 
terials of Forfarshire and Perthshire is a continuous stream, from 
three to three and a half miles wide, of boulders and pebbles, trace- 
able from near Dunkeld, by Coupar, to the south of Blairgowrie, 
then through the lowest part of Strathmore, and afterwards in a 
straight line through the lowest depression of the Sidlaw Hills from 
Forfar to Lunan Bay, a distance of thirty-four miles. No great river 
follows this course, but it is marked everywhere by lakes or ponds, 
which afford shell-marl, swamps, and peat-mosses, commonly sur- 
rounded by ridges of detritus from fifty to seventy feet high, con- 
sisting in the lower part of till and boulders, and in the upper of 
stratified gravel, sand, loam and clay, in some instances curved or 
contorted. The form of the included spaces is sometimes oval, 
sometimes quadrangular. ‘The finest examples are in the lower 
tract, which has the Dean for its southern boundary, and the road 
from the bridge of Ruthven to the south of the grounds of Lindertis 
for its northern. The Grampian boulders are throughout the same ; 
but there are associated with them masses of actinolite schist, which 
Mr. Blackadder has ascertained could be derived only from the val- 
ley of the Tay. The fragments of secondary rocks belong to the 
formations of the districts in which they occur. Though the country 
occupied by these marl-loch lakes is not traversed longitudinally by 
any river, yet it is so low, that if the transported matter were re- 
moved, a very slight depression would cause the sea to flow from 
Lunan Bay by Forfar to Blairgowrie and Dunkeld. Mr. Lyell 
therefore formerly conceived that an estuary might have extended 
in that direction, and that the till might have been drifted by 
masses of ice floated from the Grampians and contiguous hills. The 
overlying ridges of sand and gravei he thought might have been 
bars formed one after the other, in the same manner as the bar of 
sand and shingle, which now crosses the mouth of the Tay. The 
inland ridges of sand with boulders, which Mr. Lyell noticed in 
Sweden, and certainly produced under the sea, confirmed him in 
this view. These Swedish ridges are from fifty to several huadred 
yards broad, but sometimes so narrow on the top as to leave little 
more than room for a road; they are from fifty to a hundred feet 
high, and they may be often traced in unbroken lines for many 
leagues, ranging north and south. In his aceount of these ridges, 
in a memoir published in the Philosophical Transactions*, Mr. 
Lyell states his belief that they were thrown down at the bottom of 
the Gulf of Bothnia, in lines parallel to the ancient coast, and during 
* 1835, pp. 15, 16. 
