290 
ME. PEESTWICH ON THE GEOLOGY OE THE DEPOSITS 
the valleys proceeded with greater energy in consequence of successive increments in 
the mean annual temperature of each succeeding year, let us consider what other 
effects might have resulted from the operation of these causes. 
The mean annual rainfall of the South-east of England and the North of France is 
24 inches. The chief fall is in autumn, and the greater portion of it is carried off as it 
falls ; and there is rarely any large accumulation of winter snow. This fall is so small 
that it requires but a moderate excess in the fall of any one period of the year to 
produce floods which cover the whole breadth of the present valley-channels. It 
was a fall of only 3f inches* in the twenty-four hours that caused the disastrous floods 
of Morayshire and Aberdeenshire in 1829. Amongst the other remarkable facts con- 
nected with that event, Sir T. Dick LAUDEKf states that at Invercauld the small river 
Dee rose 14^ feet above the usual level, and spread 400 yards wide. A tributary stream 
cut away 6000 square yards of gravel, and spread the debris over thirty acres of land. 
Lower down, at Maryfield, the Dee rose 25 feet above its ordinary level. At Park the 
rise was 13 feet, and the breadth of the inundation not less than half a milej. At 
Murtle the river changed its channel from one side of the valley to the other, and acres 
of land were covered with gravel brought down from the upper parts of the river. The 
Findhorn rose in one place 50 feet, and in another place cut “ a new channel for itself 
for at least a quarter of a mile” §. Although, owing to the difference in the geological 
nature of the ground, the effects of such an exceptional rainfall in the south of England 
would be less than in Morayshire, it would not require any extravagant addition to the 
small rainfall of the present day to increase both the permanent volume and the floods 
of our rivers to the extent even of producing inundations more of the character of those 
indicated (at page 276) by the position of the brick-earth, or of those of arctic regions. 
Such a result might have been formerly obtained, 1st, by a direct increase in the 
rainfall ; 2ndly, by the accumulation and rapid melting of the winter snow ; or by the 
two causes combined ; and 3rdly, by the fall of rain in the spring while the ground was 
in a frozen state ||. 
The line of 35 inches rainfall now touches the north-western point of France, the 
western point of England, and the south-western part of Ireland. An advance inland 
of this line, arising from the greater precipitation determined by the low temperature 
of the land surface, would result from a general winter covering of snow — the accom- 
paniment of a climate of the character we have inferred. It may be objected that, 
judging from the fact of the decrease generally observed in the rainfall in proceeding 
Sir Dick Lauder considers, however, that the fall may have been greater amongst the hills at a distance 
of twenty to fifty miles, hut we are without information on this point. The rain for the month was 7’36 inches 
at Huntley, and inches at Inverness. 
t An Account of the great floods of August 1829, in the province of Moray, &c., 2nd edit. p. 372. 
Z Ibid. pp. 390, 391. § Ibid. pp. 38 and 104. 
|| This is of rare occurrence in this country, but when it does happen it leads to disastrous floods. Mr. 
E VAN'S informs me that the only occasion on which the valley of the Gade is flooded is when rain falls after a 
evere winter before the ground is thawed. 
