A HISTORY OF LANCASHIRE
over towards the coasts of Lancashire and north Wales it is very much greater, and in the bays and estuaries of that side the stream is very rapid and the rise of the water is very great. Thus at Liverpool the maximum velocity of the stream in the River Mersey is from 7 to 8 knots per hour, and the maximum tidal rise during spring tides is over 30 feet.
It is due to these conditions that the gradient of depth in the Irish Sea is much greater on the Irish than on the Lancashire side. Starting out from (say) Dundrum Bay on the former side we encounter the 50-fathom line at about 15 miles from the coast, and long before we reach the Calf of Man we are in water of about 80 fathoms in depth. But crossing from Fleetwood towards the Calf, the gradient is very much less, and the average distance of the 10-fathom line from the coast may be stated as about 10 miles. The 20-fathom contour is about 20 to 30 miles from land, and between the Lancashire and Manx coasts the greatest depth is not over 20 fathoms except for one considerable depression. It is a credible hypothesis that Morecambe Bay itself has resulted from the rapid eastward stream due to the meeting of the north and south tidal streams, and however this may be it seems certain that the shallow water area along the coast of Lancashire is due to erosion of the coast-line in the past, and the distribution of the débris so formed by the strong easterly and north-easterly tidal streams. The peculiarly evanescent nature of the Lancashire coastal waters is due to the shallow sea so produced, and to the great rise and fall of the tides. Twice a day practically the whole of Morecambe Bay and great stretches of the Lancashire coast are laid bare and become dry land.
Sand is the characteristic bottom deposit in the sea off the coast of Lancashire. Here and there the bottom consists of sand with varying proportions of mud, and far out at sea we find extensive deposits of calcareous matter, shells and comminuted fragments of the same, with material resulting from the denudation of calcareous rock, also deposits formed by calcareous algæ. For the most part the Irish Sea bottom is clean sand or shelly gravel, and affords good trawling ground. Only here and there do we find rough ground on which the trawl net cannot be used. The greater portion of the inshore sea bottom consists of sand or mud, with in places very restricted patches of rough stones or gravel.
We find as a result of the shallow seas and the rapid tidal streams that the sea water off the Lancashire coasts hardly ever presents that pellucid appearance which may be observed in the sea off a rocky coast, or far out from land. The rapid tidal streams stir up the bottom and cause muddy particles to be carried about in a state of suspension. River waters also carry down a considerable amount of suspended inorganic matter to the sea. There are no great rivers falling into the Irish Sea on the coast of Lancashire, but those that do exist exercise a considerable influence on the specific gravity of the sea water, which nowhere has the high density characteristic of truly oceanic water. As a general rule the specific gravity is less than 1.026, and is often very much less than that. Hydrometer readings of 1.016 have been made in the River Mersey off Liverpool landing stage, and readings of less than 1.020 in the Crosby Channel. On one occasion (Nov. 1904), I found the specific gravity of the sea water a mile or two off Blackpool to be no more than 1.021. About a week before this date there had been exceptionally high floods in
88
