90 DR. J. \V. HESLOP HARRISON ON 



and many minor streams emptied themselves into a broad and 

 rather complex common estuary, extending far inland, and 

 including the valleys now traversed by Billingham Beck and 

 others adjacent ; it is even possible that some of the low-lying 

 flats of Mid-Durham formed part of the same system. Be that 

 as it may, with the freeing of the North Sea from ice, far reaching 

 changes took place. Owing to the southward trend of the 

 tidal stream in the North Sea, in conjunction with the inability 

 of the somewhat sluggish Tees to carry away the silt borne 

 inward by the sea, enormous deposits of marine warp, which 

 form no inconsiderable part of the sandy and muddy flats 

 surrounding Tees Bay, were laid down as well as small 

 portions further up the river. Aided by the impeding action 

 of these deposits, a gradual backward filling up of the arms of 

 the old estuary occurred, thus originating the tremendous 

 stretch of alluvial soils now present ; these are composed of 

 sandy clays, heavily impregnated with magnesium and calcium 

 carbonates washed down by the streams from the Magnesian 

 limestone. 



In thickness, these clays, and any boulder clay present, 

 average a hundred feet. Beneath these lie Red (and occasion- 

 ally Grey) Marls and the New Red Sandstone which share 

 between them somewhat irregularly a further thickness of 

 1,000-1,500 feet. In the vicinity of Middlesbrough, and ex- 

 tending under the river to Greatham, these are further followed 

 by a layer of 100 feet of rock salt which, however, does not exist 

 far inland as it appears to form a saucer-like deposit, thinning 

 out rapidly in all directions ; borings indicate its complete 

 absence under Billingham and Norton parishes. 



It must not be supposed that with the cessation of the 

 greater silting-up movements just described that the flats 

 assumed characteristics approximating in any way those of 

 to-day ; such a view is far from harmonising with tlie facts. 

 Various borings near Middlesbrough, revealing the presence 

 of peat beds, tell us very emphatically of far reaching changes, 

 the causes of which we can only guess at, and the magnitude 



