332 Notes and Comments. 
streams ; hence the river system as a whole is in a compara- 
tively early stage of development. It is, however, the product 
of at least three distinct cycles of erosion :—First, the 
comparatively smooth surface of the upland is part of a 
peneplain. If its valleys were filled up it would be a plain 
sloping gently eastward with its surface cutting across the 
rock strata at a small angle. The formation of this peneplain 
probably occurred during the Tertiary era. Second, the wide 
shallow mature valleys mark the second cycle. Their shallow- 
ness indicates that the change of base level which caused 
their formation was small; and their relation to the glacial 
drift and the route of the ice indicates that they had 
reached their full development before the Ice Age. 
TERTIARY ELEVATION. 
The elevation which led to their formation probably occurred 
in the latter part of Tertiary time. The main consequent valleys 
are all of this type ; and the more important of the subsequent 
valleys are also pre-glacial, though somewhat younger. The 
Ice Age does not seem to have caused any serious changes in 
the stream lines of Teesdale ; but the extensive river terraces 
of many of the valleys probably date from the period following 
the melting of the ice. Third, the deep and narrow gorges in 
which many of the streams flow are the product of the last 
cycle of development, which is still in a very youthful stage. 
Its initiation was due to a post-glacial uplift of the region. The 
change from the second to the third cycle is readily seen in the 
longitudinal sections of the streams, most of which show very 
marked changes of slope. The fall is usually much greater in 
the lower course than in the middle. A typical tributary valley 
consists of three clearly marked sections. First, the upper course 
on the upland with a very slight valley. Second, a broad and 
shallow valley on the floor of which the stream meanders. 
Third, a gorge in the bottom of this wide valley in which the 
stream rushes along over a series of rapids. These three 
sections of the valley repeat the three types of topography 
which characterise the district as a whole, and are the results 
of the three cycles of erosion to which its present form is due. 
THE AVONIAN SHORE LINE. 
Dr. Arthur Vaughan exhibited a map showing the shift of 
the western shore line in England and Wales during the Avonian 
period, in which he gave an idea of the geography of the sea in 
which the limestones were laid down. Dr. Vaughan suggested 
that the result of the study of the ‘ zones’ of the limestone 
proved the existence in early Carboniferous Limestone times of 
a great land mass, including all Wales and the Wicklow moun- 
tains, and a continuous land mass extending through the Lake 
District, the Isle of Man, and the mountains of County Down. 
Naturalist, 
