NATURE AND MAN IN THE FORTH VALLEY. 87 
about a dozen, larger and smaller. I may be wrong, but it 
seems to me that these islands, in my own recollection, have 
increased considerably in size. All the above are within 
the tidal portion of the river. 
Above the cruive dykes, which stop the further progress 
of the tide, the Forth is navigable by rowing-boats for 
many miles, probably as far as Flanders Moss, being not 
less than 20 miles by the river above the tide limit. In 
that distance it has no islands. It is slow flowing and 
deep, with a very slight fall. 
The Teith, however, which joins the Forth half a mile 
above the cruive dykes, and contributes to the united stream 
a larger body of water than the Forth, ceases to be navigable 
a few hundred yards above the junction. It is swift 
flowing, alternately deep and shallow, and has a rapid fall. 
It contains many interesting islands, such as those at Old 
Keir, Row, Torrie, Callander, Callander Hydropathic, and 
Pass of Leny. 
The course of the river gives the opportunity for investi- 
gating the formation of mud banks, sand bars, and shingle 
beds, and of fords. A noteworthy feature of the river is 
the formation of shifting sands, which change their position 
from day to day owing to the continual contest of tide and 
current. These have been the cause of many drowning 
accidents to persons bathing. Scarcely a summer passes 
without one or more fatalities. At various parts of the 
river course appear ridges of gravel stretching to all appearance 
across the stream from bank to bank, so as to appear quite 
passable fords. There is, however, always one spot where 
there is a narrow break with very deep water and a strong 
current—a fact not apparent on the surface, and yet evident 
enough, if one reflects that the large body of water finding 
its way down the stream could not possibly be all passing 
over the shallow bar. I know of at least one case where 
a man fishing stepped suddenly into deep water and was 
carried rapidly down by the current and drowned, These 
gravel bars suggest interesting questions. For instance, at 
Abbey Ford, on the east side of Cambuskenneth Abbey, 
at low spring tide, there are several acres of gravel and 
boulders laid bare in the bed of the stream. Where have 
