



318 STRUCTURAL AND FIELD GEOLOGY 
at various levels above the sea (see Plate LIV. 1). They 
may consist of ordinary beach materials—gravel and sand 
with rolled and broken sea-shells, etc. Along the margins of 
estuaries they often form wide flats, composed for the most 
part of finer materials—sand, clay, loam, silt, etc. On our 
more exposed sea-coasts the raised beach-lines are frequently 
mere platforms and ledges, which have been sawn out of the 
solid rocks by the sea. Many old beaches are backed by cliffs, 
at the base of which sea-worn caves not infrequently appear. 
In Scotland there are three “ancient sea-margins” which are 
particularly noteworthy. They occur at heights of 100 feet, 
50 feet, and 25 feet respectively. The highest is the oldest, 
and is best developed in the basins of the Forth, the Clyde, 
and the Tay. It is composed largely of laminated brick-clay, 
together with fine sand. Scattered through these deposits 
occasional stones occur, and now and again large erratics are 
even common. The “beds not infrequently yield) mene 
species of shells, etc. The observer will find it interesting to 
follow the 100 feet beach or terrace up the valleys until it 
merges into terraces of ordinary fluviatile shingle and gravel. 
When the latter are traced further inland into the mountains, 
they will eventually be found to interosculate with fluvio- 
glacial gravels and terminal moraines. 
The two lower terraces are of later date, but their geo-. 
logical history has not yet been so fully worked out as it 
- deserves to be. 
LACUSTRINE AND FLUVIATILE DEpoOsITS.—The sites of 
old lakes are readily detected. They invariably occur, as might 
have been expected, in hollows and depressions, and usually 
form level meadow-lands. Their margins, as a rule, are well 
defined. The observer should never miss the opportunity of 
examining any cuttings in which the old lacustrine deposits 
are exposed. Very often the immediate surface is occupied 
by peat of less or greater thickness—or several layers of peat 
may be interstratified with sand or silt. The peat may 
consist entirely of plants which still grow in the neighbourhood. 
Now and again, however, Arctic plants have been detected in 
the basal part of the peat or in the immediately underlying 
silt or clay. Sometimes, also, traces of Arctic animal life are 
met with in the same deposits. This proves that some of 

