292 BIRD-LIFE OF THE BORDERS 



in bringing about the altered physical conditions which 

 then prevail. Over these dead-level expanses of wet sand 

 there is spread for miles a thin covering- — a mere film — of 

 salt water, either left by the tide or blown up by the wind. 

 In severe frost this film freezes into a sheet of ice, so thin 

 and elastic (or pliant) as to rise with the succeeding tide, 

 unbroken. With every ebb its thickness increases ; then, 

 perhaps, comes a snowfall, and the surface of the ice is 

 covered three or four inches thick with snow. This 

 freezes to the ice below, and, in fact, forms a compact 

 and homogeneous mass with it. Other snowfalls follow, 

 each thickening the growing ice, till in the course of a 

 three-days' frost there is formed on the sand-flats what 

 can only be described as a vast ice-field, perhaps hundreds 

 of acres in extent, and anything from a foot to a yard in 

 thickness. The weight of the ice, when lifted by the 

 flood-tide, causes the solid field to split and crack up, 

 the chief breakages occurring along the outer side, where 

 the water is deepest and the influence of the tideway 

 most felt. The whole ice-field also tends to slip outwards 

 on the ebb ; while, so long as the frost holds, the process 

 of congelation and the creation of new ice continues afresh 

 with each tide. Thus very great quantities of ice are 

 carried off the flats by every ebb, while the places left 

 vacant to landward are reoccupied by a further generation 

 of glacial supplies. 



Even in a frost of but a few days' duration the quantity 

 of ice thus daily set afloat by the tide is very considerable. 

 But when, as happens in such extremity of cold as we ex- 

 perienced in the winters of 1878-9, and again in January 

 1 88 1, the frost continues unbroken for weeks at a time, 

 and with a temperature approaching zero, the phenomena 

 created thereby are indeed incredible, save to those who 



