The Essex Rock & Mineral Society's
field visit to:
Birch Quarry, near Colchester, Essex.
4 August 2002
Birch
quarry is a working sand and gravel quarry but fortunately silent on Sunday's.
The sands and gravels are believed to be from the pre-glacial course
of the River Thames and the 'Boulder Clay' from the last Ice Age!
Members
starting the day!
Gerald Lucy's book, ESSEX ROCK, has an excellent chapter on the geology of the period. Essential background reading and positively recommended!
In
this view you can see graded piles of sand and aggregate. Quite handy
for those fist sized cobbles. These were originally river deposits, the
stones tend mostly to be of flint.
However, it is a handy source to scour through looking for that odd erratic from further afield.
Rock like the 'Hertfordshire Pudding Stone' and 'Fossil Wood' can usually be found in the course material. The secret to collecting from these piles, is to pick up the cobbles that look out of place.
Especially
in a working quarry it can be difficult to know where to look. Here is
an in situ exposure showing various features. By carefully examining
these (when found) you can get a feel for what was going on in those times.
Sand looks...well... like sand! But if you look with understanding at samples, you can tell whether they are desert, fluvial or beach deposits, just by examining their characteristics.
Another important point about being able to interpret sedimentary structures, is that it can help identify the most likely spot to find fossils. In this case, if one could have found evidence of previously exposed, sandbars, land or riverbanks, you may have been lucky and found fossil mammal remains! Also, from the grain size of sand samples, it is possible to estimate the river's rate of flow at the sampling spot.
Members
having a break and discussing their finds!
(Don't ask me what they found, I don't recall anyone bringing any finds from this location to the Members' evening. As for the trip in general, you can expect the official report to appear in the next newsletter. All I remember hearing was that about sixteen members attend from the Essex Rock & Mineral Society and three from the Essex Filed Club. Only a few went back to Bob Burton's for tea and sandwiches. Those that didn't must be complete fools!)
Very many thanks to Bob & Sandra Blackburn for the pictures. (The above views are a selection of those received. The others, not shown, have been been archived for future reference.)
( © R Coleman 19 August 2002)
(Printing: Document optimized for A4 format
with all margins set to 10mm)