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7i 



Dorset during the Glacial Period. 



By Hy. Colley March, M.D., F.S.A. 



Read before the Geological Section in December, 1909. 



In some parts of Europe there are evidences both of inglaciation 

 and of deglaciation, and in others of deglaciation only. The 

 proofs of inglaciation are concerned with dynamic pressure, 

 pressure with movement on the land surface. Such are the 

 polished and grooved or striated rocks over which the glacier slid, 

 bearing its impacted boulders and rock fragments • and the contour 

 and composition of the moraines which differ distinctly from beds 

 of debris brought down by torrents. 



Moraine- stuff, according to Geikie, is angular rubbish which 

 has fallen from cliffs and crags, and is carried along by glaciers. 



Boulders are large erratics ; any transported mass, as clay 

 and chalk, or large water- worn blocks of stone. — 1. 



Boulder-clay denotes the stiff unlaminated clays of the 

 " Drift," containing numerous boulders and pebbles. These clays 

 have been produced either by floating ice dragging on a sea-bottom, 

 or by the dynamic pressure of glaciers on land. The clay has the 

 colour of the formation from which it was derived — red, from old 

 red sandstone, dark blue from coal shales, and creamy from 

 oolite and chalk. — 2. 



Boulder-clay often consists of materials graduating downwards 

 from the smallest stones, through sand-grains of various degrees of 

 fineness, down to rock-meal. — 3. 



Stones in the boulder-clay are usually oblong, have one or 

 more flat sides or soles, are smoothed or polished, and present 

 rounded edges. When they consist of a fine-grained enduring 

 rock they are generally stricted. — 4. 



An ice-sheet is the deep mantle of snow and ice which covers 

 land. It may remain stationary as sedentary ice, but more often 

 it either descends as a glacier to lower levels where it is melted, or 

 creeps out to sea and becomes an iceberg. An example of sedentary 

 ice can be observed in Alaska. About a mile from the sea the ice 

 forms a ridge two miles wide and 250 feet high, overlaid with clay 

 and vegetable mould. The surrounding country has no high land or 

 rock-hills from which a glacier might have been derived. The 

 continuity of the mossy surface shows that the ice must be quite 

 destitute of motion. It takes upon itself the functions of a regular 

 stratified rock. — 5. 



1 — -Melland Reade. Q.J.G.S., xxxviii., 220. 

 2 — David Page. — Hand-book. 

 3— Percy Kendall. Glac. Mag., ii., 145. 



4 — Geikie. 

 5— W. H. Dall. Q.J.G.S., xxxviii., 234. 



