Conglomerate and Gravel Deposits. 395 



of most shingle beaches, particularly when they defend tracts 

 of flat country, is bounded by a line along the edge of the 

 beach ; above this line the beach generally makes a consider- 

 able angle with the sands, in cases of sandy flats. 



In cases where shingle beaches are not entirely quitted by 

 the tide, saudy, shelly, or very fine gravel soundings are 

 commonly obtained a short distance from the shore, unless the 

 bottom be rocky, in which latter case it is generally a mixture 

 of sand, rock, or fine gravel* and shells. In fact, if the present 

 continents or islands were elevated above, or the sea depressed 

 beneath, the present ocean level, shingle beaches would be 

 found to fringe the land, but not to extend far seaward. 



It is but rarely that the pebbles on shingle beaches are found 

 to have travelled considerable distances, even along shore ; in 

 the Chesil Bank indeed, — that extraordinary ridge of pebbles 

 about sixteen miles long, which connects the Isle of Portland 

 with the main land, — the shingles seem to have travelled twenty 

 or thirty miles from the westward. This bank is remarkable 

 on many accounts, and among others for the power the sea 

 has exhibited of heaping up a barrier against itself, even when 

 not backed by land, provided it has two solid resting places 

 for each end of the bank. It also appears tha* the shingles do 

 not travel from the bank; for Portland Roads have a bottom of 

 clay, the continuation of the Kimmeridge clay of the base of 

 Portland and the Ferry Point, affording one of the best holding 

 grounds for vessels in the Channel ; and the bottom to the S.W. 

 of the bank is sand, fine gravel with shells, or rock.t 



Single beaches are generally formed on the sea shores under 

 consideration, from the harder parts of the neighbouring coasts 

 being destroyed by the joint action of atmospheric agency, 

 land springs, and the sea. The softer portions are soon washed 

 away, and even the harder, first forming the shingles, are event- 

 ually ground down into sand. It is, however, by no means un- 



* These gravels are generally fine, very different in size from the common 

 shingles of beaches. It might be supposed by persons unaccustomed to take 

 soundings, that the gravels marked on charts were coarse, resembling shingles; 

 but in general such gravels do not exceed the size of a nut, and are most com- 

 monly smaller. Such fine gravels are very frequently mixed with shells ; and no 

 soundings are more common on coasts, particularly our own coasts, than gravel 

 with shells, sand and gravel, and sand and shells. 



1 \ This bank also possesses considerable interest in another point of view. 

 The hills behind the bank are composed of clay and loose rubbly or slaty lime- 

 stones (Forest marble, Cornbrash, Oxford clay, Oxford oolite, and Kimmeridge 

 clay,) which, if not protected by this mass of shingles, would soon be swept 

 away before the heavy seas rolling in from the Atlantic, and breaking with so 

 much fury on this coast. That they have not been thus attacked is evident, for 

 the large rounded forms of the hills and dales are only here and there marked by 

 little cliffs, cut by the water intervening between the bank and main-land ; it 

 therefore seems fair to conclude, that since the existing order of things in the 

 Chesil Hank has existed, and that the main-land behind it has not, since it ac- 

 quired its gentle midulatory form, been attacked by the furious waves from the 

 Atlantic. 



