The tops of the upper chalk hills, about Whips nade ,and Luton, have in general a covering of alluvial red clay mixed with broken flints, sometimes in such regular layers in the clay, as to have been called strata of flint, and often in such quantities, as after rain to cause the ploughed fields to appear like gravel heaps. On others of the hills of this district, particu larly near Kingsworth, great quantities of the small black chert pebbles of the London clay are found,' (See Phil. Meg. vol. xxxv. p. 131.) with broken flints and larger chcrt pebbles in the vallies.
The strata of Bedfordshire have an easy and pretty regular dip towards the south-east, at the rates of 1 in 50 to 1 in 80 perhaps. The uppermost stratum which appears in Bedfordshire, is a thick bed of chalk, with numerous layers of flints throughout its whole thickness; but near the top they are closer together, and the nodules larger, and the intervening chalk in this situation is more free or soft, and is alone fit for the chalking of lands and the making of whitening, on account of its friability: it is also the purest car bonate, and contains less silex and other heterogeneous mixtures, than the lower chalks do. This upper chalk advances no farther northward or north-west than the hills on each side of Luton, and those south east, south-west,, and west, of Dunstable.
The hard or lower chalk next succeeds, in which no flints are found ; but the chalk increases in hard ness, and the quantity of the fine grains of silex which it contains, as we proceed downwards, until near its bottom a stratum little different in appear ance from those above it is found, which proves to be a very durable freestone, when seasoned or dried gradually without' suffering frost to reach it, which, when fresh dug, would otherwise shiver it completely, to pieces. At Totternhoe, north-west of Dunstable, there are immense and ancient workings after this stone, which will stand tire, as well as weather, in a vertical wall. Woburn-Abbey, the New Swan Inn at Bedford, and many other good buildings, are faced with this stone ; and the window-jambs and ornaments of most of the churches in the midland counties are made of it. The hard beds of chalk above this Tot ternhoe stone, such as are seen in the second new road .now cutting at Chalk, or Puddle-hill, near Dunstable, are called hurlock, and make a very good lime for building, probably on account of the great quantity of silex it contains ; and it is from these hurlock beds, all the way from near Eaton-Bray to Baldock,. that the parts north of this for several miles are furnished with lime, except a little for plasterers work or whitewashing, which is brought from the softer and whiter beds of the upper chalk, to the south of the range made by the hard chalk and hurlock. The large quantity of silex, which some of this hurlock con tains, may be one reason, joined with the dearness of fuel to burn it, that lime has been so little used or even tried of late years, on the clays or sands north of the chalk hills in Bedfordshire, under the idea of its not repaying the expense. The upper and lower
chalk are, it is said, together about 400 feet thick.
The chalk marle is the stratum which succeeds the chalk, and on the surface when wet, makes very te nacious white or gray clayey soil ; but when dry, the white colour is seldom preserved, yet a dark-coloured loam results from its decomposition, although when fresh dug into, the strata might almost be mistaken for rubbley or broken hurlock. The basset,. or north western edge of this stratum, was lately exposed by the widening the London road through Kates-hill, at th'e south end of Hockliff town. • In proceeding from this northward, along the road towards Towceter, nothing but thick masses of allu vial clay are seen, until near Sand-house at 40; miles from London ; and in the other road through' 'Woburn, the same alluvial covering prevents any ob servations being made on the strata, until within a quar ter of a mile of Woburn town the same sand makes its appearance, from under the alluvium : and it is not a little remarkable, that the distribution of this allu vial clay is so complete across the country, all the way from Billington to Cockayne-Hatley, as shewn by the blue colour in Mr Batchelor's map above refer red to, as entirely to conceal from the knowledge and use of inhabitants of Bedfordshire, the remarkable limestone strata of Aylesbury, and several others, which no where burst through or show themselves. from under the alluvia, the " golt" of the Rev. Mr Michell. See the Phil. Mag. vol. xxxvi. page 103: The Woburn sand is a name for a series of fermi; ginous sand strata, about 170 or 180 feet thick, whose basset crosses Bedfordshire from Leighton Busard to Potton, as already mentioned, when speak ing of the alluvium and peat upon it. In most parts this sand near the surface is cemented by an oxide of iron, into a dark red sand-stone, the grains of silex in which arc of very unequal sizes, even in the same specimen. This stone is called car-stohe in some places, and was for many years, previous to the roads in Wo burn being undertaken by the late Duke's agent, the sole material used on the roads ; and the grinding of it, and subsequent. washing away of the ferruginous ce menting matter by the rains, occasioned two-thirds in length of those deep, loose sandy road's, (which were the remark and terror of travellers who valued their horses, under the name of the Woburn Sands,) across tracts of alluvial clay, where not the least sand is to be found but what has been brought thither, when in the form of car or sand-stone. IL is much to be lamented, that this absurd practice still continues in the parishes of Wavendon and Brough, ton, between Woburn and Newport Pagnel, instead of searching for and clean sifting the quartz and flint gravel, which is to be had in sufficient plenty.