16 Tlie Blackwater Valley, Essex. 



The broad gentle slope traversed by these rivers consists of 

 Boulder Clay, at the surface, from ten to eighty feet thick, 

 underlaid by sand and gravel, rarely more than tlm-ty feet 

 thick, and sometimes altogether absent, and this by the 

 London Clay, the limits of which are near a line through 

 Thaxted and Bishop Stortford. To the north-west of this 

 line is a narrow belt of the Lower London Tertiaries, fol- 

 lowed by the Chalk, the Boulder Clay and subjacent gravels 

 generally concealing these older beds. On the flanks of the 

 Ter Valley occur patches of clean brick-earth overlying the 

 Boulder Clay, but possibly constitutmg only the upper pai*t 

 of that formation, and not separated from the remainder by 

 any important interval. 



Between Witham and Feering the Boulder Clay, which 

 j)reviously only formed the i)lateau between the Guith and 

 Blackwater, descends suddenly into the valley, not only cut- 

 ting off the gravels that originally lay below it, but passing 

 below the general surface of the London Clay, as shown in 

 the valley bottoms at Coggeshall, Faullvbourn, &c. The 

 Boulder Clay only rises a short distance up the flank of the 

 Tip tree ridge, the sides and crest of which are coated with a 

 considerable depth of sand and gravel. 



A part of Tiptree Heath having been fixed upon as a good 

 site for the second County Asylum, an artesian well was 

 resolved upon as the only satisfactory source of water supply. 

 Calculation from the nearest of previous wells showed that 

 there was some local disturbance. The Chalk surface 

 descends fi-om Braintree to Witham at the rate of 21*23 feet 

 per mile, giving depth at Asylum of 495 feet, whilst it rises 

 from Maldon and Heybridge at 74 feet per mile, giving 

 depth at Asylum of 194 feet. 



Three iDossibilities presented themselves : — 



1. There might be a gentle roll over of the beds. 



2. There might be a powerful undulation or reversal of 



the beds. 



3. There might be a fault thro whig down the beds to the north. 

 The boring proved the last two of these contingencies to 



be combined. 



