21 
of this great tableland. The former three unite near Heasand- 
ford, and the effect of a series of convergent rivers of that 
type, in gradually lowering the country, is well seen on the 
hillsides which slope down towards that part of Burnley from 
Haggate, Extwistle, and Worsthorne. 
The various points outlined above were illustrated by a 
specially prepared series of lantern slides, including maps of 
the whole basin, diagrams of the rate of fall of the various 
streams, diagrams illustrating various phases of special 
river-work, and diagrams of the capture of one stream by 
another which possessed some natural advantage. 
A special feature of the district is the remarkable modifi- 
cations produced in it by the ice-sheets and overflow of the 
glacial period. Some valleys have been more or less blocked 
up, and the rivers are now engaged in re-excavating their 
old valleys. Such is the case, for example, with the upper 
Don at Thursden. Other features connected with this period 
are the curious and striking gorge-like valleys in the confines 
of the district which have no river at present competent to 
erode them. Such are the famous Cliviger Gorge, the Foul- 
ridge Gorge of the Craven Gap, and what the lecturer styled 
the ,‘ Bronte Gap.” The latter is the clean-cut, rough-shaped 
valley seen on the sky-line as one looks from Colne rowards 
the Bronte country. These valleys were probably produced 
by long-continued overflow of water from the lakes which 
were formed by the dammed-up drainage of the Calder system 
by the long persistent northern ice-sheets. They form a very 
interesting feature of the district, and they do not seem to 
have been explained until recently: 
