147 



NOTES ON A CHARA AND SHELL-MARL DEPOSIT AT 

 HAWES WATER, SILVERDALE, LANCASHIRE. 



By J. DAVY DEAN and J. WILFRID JACKSON. 



(Read before the Society, December 14, 1904). 



Plate III. 



For some time past we have been investigating the vast deposit of 

 shell-marl in this neighbourhood with a view to ascertaining the 

 number of species it contains. The Hawes Water under discussion 

 lies to the north of Silverdale station, and must not be confounded 

 with the larger Hawes Water near Shap. 



There are two tarns, the largest of which occupies an area of about 

 twelve acres, and is connected with the smaller tarn, called Little 

 Hawes Water — a mere pool compared with the other — by a narrow 

 stream. Both tarns are fed by the various drains from the rising land 

 round about. They are reputed to be of great depth, and viewed 

 from the west side, where the shore shelves out and then appears to 

 drop almost perpendicularly, they certainly look it. We had intended 

 to obtain a boat for the purpose of ascertaining the exact depth by 

 soundings, but were unable to do so. 



With the exception of the southern end, the two tarns are shut in 

 by high land, gradually rising from about twenty-five feet above sea 

 level to ninety feet or more, with higher altitudes here and there, such 

 as Challon Hall Allotment, 150 feet, and Trough Plantation, 240 feet. 

 The rock of the surrounding district is mountain limestone. At the 

 south end of this bath-shaped depression it is more or less open, and 

 here a small stream issues from the large tarn, and, passing the sides 

 of Hawes Water and Leighton Mosses, receives their numerous drains 

 on its way to the sea. 



The deposit, consisting of a mass of comminuted chara stems with 

 dead shells scattered indiscriminately through it, occupies a great part 

 if not the whole of the basin described above, and it is evident from 

 this that the tarns had formerly a much larger extent than at present 

 and might possibly have been united. The severance of these, and 

 the narrowing of their limits, may have been brought about by the 

 usual process of the gradual filling up of a lake, partly by the washing 

 of detritus down from the adjoining slopes, and partly by the succes- 

 sive generations of chara decaying and falUng to the bottom along 

 with the numerous dead shells of the mollusca inhabiting the lake. 



The marl is, for the most part, covered by a bed of peat ; a good 

 section, however, presents itself at the north end of the large tarn, 

 where no peat rests on it, and where it forms a series of low abrupt 

 cliffs, from three to five feet high, encircling almost the whole of this 



