Its flint known as banded flint.
There not fossils, its just the way some flints developed under certain conditions.
I have a collection of them in my garden & they are quite easy to find at some chalk cliff beaches.
I know what you're talking about, and I respect your field experience, but I beg to differ in this case. To my eye, the lines/bands are too regular/progressive and the lower line where they end is too discrete. I suspect an imprint of a broken bivalve valve, or even a siliceous replacement of one (all within the gamut of banded flint, sure).
[For reference, I too have some experience of chalk and flints, having lived on the chalk of the North Downs for 18 years before going to study geology at Liverpool Uni for eight years.]
I also thought from the first image it may be bivalve related but the second photo changed my mind .
Spent some time looking at it again & i still see things the same way & reach the conclusion that its banded flint, even feeling more certain than before now, so with all respect we may just have to agree to disagree over this one, for now at least.
Posted some photos of some examples i have & from looking at these i can see that banded flints vary in many ways such as thickness, spacing & shape of the bands. Also in that the banding does not always end abruptly & sometimes this is more discrete than whats seen with the majority of examples, i have one specimen where the banding almost joins up, & has almost no defined edge along the length of the banding on either side.
One other thing that may tie this i.d to banded flint is that every specimen i have seen always has a red/brown iron stained? coating? to its surface. I think the flint in question is a little sun bleached but also seems to have areas of the same mineral coating.
I have also lived all my life just a few Min's walk from the Upper Chalk cliffs of Thanet. Most days i go out for a few hours with my dog & of cause fossil hunting at one of the chalk locations in Kent.
Thanks for the discussion. Great specimens and photos.
You make an interesting observation correlating iron-colouring with banding. Chemistry certainly plays a part, so I see no reason in principle why it might not be a valid (non-coincidental) correlation. It makes me think of rocks I know locally in Cyprus showing gypsum/anhydrite alteration, which is accompanied by an increase in volume, which results in a regular buckling of the strata. In one place an exposed bedding surface looks almost like giant bubble wrap (the bubbles being perhaps two metres across). (It is non-structural deformation.)
Looking at your specimens was enlightening. I notice the banding tends to be across elongated parts of the flints. I have to think that suggests formation as the flint was extending preferentially in that direction (rather than in all directions, which would have formed a more equant shape). 'Preferentially' may mean quicker or/or more frequently. That may have been due to anisotropy in various things: stress regime, rheology (and influenced by fossils or old burrows, perhaps), pore fluid chemistry, etc. Whatever mechanism was at play, I can conceive that flint-front propagation may well have been periodic, and hence liable to give rise to banding (like wood becomes banded follwing its growth at different speeds and with different structure through the seasons). Regularly repetitive structure can arise spontaneously under various circumstances, eg. ripples on the bed of a steadily flowing stream, but they are more likely where, in some sense, there is an increased rate of flow/propagation/etc That's been documented elsewhere in some books on patterns in nature.
Now, in doing a bit of Googling, I came across this wonderful example.
[http://en.wikipedia.org/wiki/Striped_flint, photographer unattributed]
The spherical shape is due to crafting by man into a decoration; ignore that.
This has changed my view on banded flint!
I have never seen banded flint structures so well in 3D before. It is so intricate. It even includes an area very much like the banded part of the flint specimen we're debating here. And it clearly is not fossil (though we can't be sure about what appears to be the nucleus).
Undoubtedly, fossils do occur in flint, and they may aid its nucleation and growth, but I suspect in future my views on these things will be more aligned with yours, Keith. Thank you for inducing me to think about this.
I hope this has proved interesting for you.
For ref. for anyone reading this thread: