SINKING OF THE LAND IN CAMBRIDGE 401 



Botanic Garden. From it run the lower spurs which furnished 

 the dry area along which streets and houses have crept into 

 the great bend of the river, which, subject to the above men- 

 tioned limitations, strategically and commercially determined 

 the position of the ancient town. 



The Recent Subsidence of the Valley of the Cam. 



In all such enquiries as that which is now engaging our 

 attention we are accustomed to regard the level of the land in 

 this country as fixed and permanent for all historic time, and 

 at a little distance above sea level small earth movements of 

 elevation or depression would have very little practical effect, 

 and would be very difficult to prove. But, when we are 

 examining deposits near sea level, in an area where the conflict 

 between the sea and the upland waters is still going on, a rise 

 or fall of a few feet makes a great deal of difference in the 

 issue. For our present purpose it is unnecessary to enquire 

 whether any of this change in the relative position of the land 

 and water is due to actual movement of the land or to the 

 heaping up of the waters by wind or ocean currents to which 

 differences of many feet may in some cases be referred. 



We have evidence in the valley of the Cam of a depression 

 in pre-glacial times which went on continuously or inter- 

 mittently through the period of the deposition of the gravel in 

 which the remains of the Mammoth occur, was continued after 

 the formation of the recent river-silt, and, for aught we know, 

 is going on still. 



Under ordinary conditions, such as we have here, a river 

 cannot scoop out its bed below sea level. If then we find a 

 river channel extending downwards below Ordnance Datum, 

 i.e. mean sea level, we may safely infer that there has been a 

 subsidence of the area since that channel was formed. 



The date of the erosion is approximately indicated by the 

 deposits which fill the trough. Now when trial-holes were 

 made in the Trinity College Paddocks, with a view to the 

 extension of College buildings over the area, old river courses 



