80 



THE SEVBEN TUNNEL. 



softer ; so that at low water tlio level of the river bed is 

 eight feet lower than on the English Stones proper, and a 

 shallow low-water basin is left by the tide, some throe or 

 four feet deep in the deepest places, and called the Salmon 

 Pool. On the other hand, the Tunnel gradient, rising at the 

 rate of 1 in 100 from the Shoots, comes nearer to the bed of 

 the river there than at a,ny other point. When the heading 

 was enlarged into the full-sized Tunnel, the Severn water in 

 its muddy state did find its way through into the Tunnel be- 

 low, and for a time flooded the sea wall portion of the works. 

 On examining the river bottom at low water, a large hole 

 was found, and the contractor then cut two wide gutters to 

 drain the Salmon Pool, and afterwards filled the hole on 

 top with bags of clay. This enabled him to build tlio arch 

 of the Tunnel under that ground, and to make all secure 

 The same thing occurred also a few chains further on, and 

 was got by in the same manner; but the ohiy in tlio hiigs 

 gradually ran. through the pipes with tlie water, and the 

 holes had eventually to be filled with gravel and coarse 

 sand, which stopped the leakage from the Severn. The 

 spring water from, below, however, continues to flow into 

 the Tunnel in many places under the Severn, a,nd is of a 

 strongly mineralised character, some of the springs colouring 

 the walls of the Tunnel a bright red, and others a jest blaclc. 

 One of these black waters has a strongly sulphureous smell. 

 They are also, as a rule, saline to the taste. 



In making the great cutting on the Plnglish side through 

 the Severn marsh lands, some cui'ious discoveries were 

 made. These lands are all level and unbroken on the 

 surface, and were evidently formed at some period by 

 deposit from the Severn. The lowest stratum, lying on the 

 new red, was coarse gravel, with large masses of old red 

 conglomerate, and of mountain limestone, deposited hero 



