THE PHYSICAL GEOGRAPHY OF THE PAST. 
249 
find a mass of pure or almost pure limestone of organic 
origin that the deposit was formed far away from land, or at 
any rate in water absolutely free from the influence of streams 
bearing their freight of mud seawards. 
Such a mass of limestone is that of Central and North Derby¬ 
shire, and we are justified consequently in taking the first 
step in our reconstruction of the physical features of the 
Lower Carboniferous period, by assuming that this immensely 
thick mass of almost pure limestone marks the position of an 
area of deep and perfectly clear water. 
When this mass of limestone of the Pennine Range is 
traced along the country to the north, we lose sight of it in 
the neighbourhood of Castleton, owing to its disappearance 
below the Yoredale Shales and Millstone Grit. When it is 
once more brought to the surface in the neighbourhood of 
Skipton, in Yorkshire, by the east and west plications already 
referred to, it shows some decided indications of altering its 
character, for we now find it containing several beds of shale, 
or hardened clay. Still, however, the limestone predominates, 
but the deposit has not quite the pure character of the lime¬ 
stone further south. A little further north still the shales or 
clays become thicker, and the limestones thinner, and, at the 
same time, the beds of limestone become divided by beds of 
sandstone. This progressive change continues right into 
Northumberland, where the massive Mountain Limestone 
and Yoredale Series of Derbyshire are found to be replaced 
by a mass of sandstones, shales, and thin limestones, con¬ 
taining in the upper part as many as seventeen seams of thin, 
but workable coal, all deposits of shallow water origin. 
Now this extraordinary but gradually progressive change 
in the character of the beds when traced laterally, can only 
be satisfactorily explained in one way, when we bear in mind 
what we may call the physics of sedimentation. As we 
proceed northwards, we are leaving behind us the deep sea of 
Lower Carboniferous times, with its clear water and limestone¬ 
building creatures, and are approaching, through gradually 
shoaling water, the old coast line, and the mouths of the rivers 
which brought down from the old Carboniferous land the sand and 
mud which now form the sedimentary deposits. 
The exact position of this old coast line is indicated by a 
bed which lies at the very base of the Carboniferous rocks of 
the North of England, but which is, of course, not met with 
in the Midlands. It is what is known as a Conglomerate, a 
rock-like mixture of sand and rounded pebbles. It is, in fact, 
a consolidated and fossilised sea-beach, and we find it abutting 
against the old shore formed by the Cheviot Hills, from which 
most of its rolled fragments have been derived. 
