A HISTORY OF LANCASHIRE
addition to those mentioned. Sections in the Boulder Clay can be seen almost everywhere, and are especially marked on the coast, where at times they form cliffs, as at Blackpool, from 40 to 70 feet in height.
One interesting feature of the Lancashire drift which still requires working out is the occurrence of broken and comminuted shells, and isolated valves. These are found even in the inland clays. Amongst others, the writer has found valves belonging to species of Cardium, Mactra, Mytilus, and a portion of the test of an Echinoderm. Foraminifera also occur.
In many places the drift can be divided into three parts, a middle division of sand being intercalated between lower and upper Boulder Clays, or Drift. Pockets of sand, sometimes of large size, at times occur interbedded with the clays.[1]
Post-Glacial Deposits.—To this category belong the extensive peat deposits of the moorlands and plains, which are often of considerable thickness, especially in the 'Mosses,' as Chatmoss, etc., and contain trunks and stumps of trees, sometimes in such profusion as to indicate that many districts and even hills were densely wooded instead of bare and bleak as we now see them.
Here also must be placed the banks and deposits of Alluvium at the mouths and along the sides of many of the rivers, and the extensive dunes and sandhills which are so striking a feature of the coast between the mouths of the Mersey and the Ribble, near Blackpool, and at Walney Island.
In the neighbourhood of Fleetwood, Poulton, and Blackpool, these later deposits have been classified by the officers of the Geological Survey as follows:—
| Post-Glacial | Recent | Blown Sand. | ||
| Upper 'Cyclas' clay, sand, etc. | ||||
| Upper 'Scrobicularia' clay. | ||||
| Marsh clay and tidal alluvium Peat. | ||||
| Pre-historic | Lower 'Cyclas' clay. | |||
| Lower 'Scrobicularia' clay. | ||||
| Presall Shingle. |
A somewhat similar division holds good for the district around Southport, the place of the Presall Shingle being taken by the Shirdley Hill Sand and Lower Peat.
BLOWN SAND
Sandhills are forming so extensively along the Lancashire coast that a few words need to be written respecting them. The set of sea currents is such along the coast from north of Liverpool to Fleetwood that almost continuous sandy beaches are formed. Indeed, these have accumulated in some places to such an extent that the sea appears to be retiring from the land. This is well seen at Southport, where marine lakes and promenades take the place of what was once open beach swept by every tide. The exposure of the sandbanks at low tide to the sun results in the upper layer of sand becoming dried, when it is easily moved by the wind and swept inland, where it collects against any obstacle, such as fences or buildings, and accumulates until it at length overtops them, and falls over upon the other side. In this way a low eminence is formed, which is continually being added to on the seaward side and as continually being reduced by the surface being carried further inland. In this way an extensive belt of arable land has been covered over, and the encroachment has become so serious that vigorous attempts are made to stop its further progress by planting 'starr-grass,' Psamma arenaria, and Ammophila arundinacea, whose long-matted roots hold the sand together, whilst the leaves protect the surface. Southport is entirely built upon blown sand, which can also be seen inland behind it.
At Formby the sandhills are three miles in width, although it is stated that none existed so late as 1690, the whole deposit having been formed since by the silting up of the then Formby Harbour, and the formation of a sandbank against the land, from which the loose sand was carried landwards. Between Formby and Birkdale, near Southport, many farms have been entirely covered up within the last hundred years, and houses completely buried.
The sand often contains shells and shell fragments, which have been also wind-borne, and, these decaying, the carbonate of lime of which they consisted becomes dissolved in the acid-laden rain, and, being afterwards reprecipitated, it serves as a cementing material to the sand, which thus becomes solidified, and even impermeable to water. Between Fleetwood and Rossal the sand is extremely large-grained.
- ↑ There is a considerable literature dealing with the Glacial Drifs, and we are indebted especially to Mr. T. Mellard Reade, Mr. R. H. Tiddeman, and Mr. C. E. De Ranee for records of fects and explanations.
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