Dear Readers, it felt very strange to wake up this morning and realise that I had actually managed to get everything done that I wanted to get done yesterday, but I didn’t have long to cogitate as we had an organised Geology walk in Coldfall Wood today. It was on the subject of geology, which for me is the unheralded crux of ecology – what underlies the soil determines so many things, from the soil organisms that will thrive to the plants that will grow, as anyone who has tried growing chalk-loving flowers in a London clay garden will tell you. The walk was led by Diana Clements, who is currently revising her book ‘Geology of London’, which is well worth a look for anyone even vaguely interested in the deep history of our area.

We looked at the main rock types in the area – London clay, Dollis Hill gravel and glacial till. Diana’s walk rather cleverly takes us through the three stages of the history of the wood as reflected in their geological history, and I for one will never look at the them in quite the same way again.

The rather unprepossessing bit of the stream above shows that the banks are London clay, and Diana had a box full of fossils found in clay, from North London molluscs, shark’s teeth, palm seeds and magnolia seeds. The clay was first laid down about 50 million years ago, when the climate was probably tropical (though the magnolia seeds may suggest at least some seasonality). Magnolias are ancient flowering plants that are pollinated by beetles, as there were no specialist pollinators about at the time.

Next it was off to the wet woodland for a look at the Dollis Hill gravel. The Thames used to run to the north of London, through the Vale of St Albans and then into the North Sea at Clacton, until it was diverted by the glaciers of the Ice Age. Some of its tributaries flowed through what’s now Coldfall Wood, depositing gravel as it went. You can find all sorts of interesting things in gravel, including quartz and the flinty Lower Greensand Chert.

The bed of the stream into the wetland area is full of gravel.

And finally, there’s the glacial till. One finger of the last glacier of the Ice Age (which retreated about 400,000 years ago) reached as far south as Coldfall Wood (and also Hornchurch in Essex for anyone who lives in those parts). As it retreated it ‘dropped’ all the rock fragments that it was carrying (to a depth of 14 metres in Finchley), and simultaneously excised deep gullies as the water in the ice sheet melted, while the surrounding soil rebounded after being compressed by the ice. No wonder the woods are so undulating, although they’re probably less so than they used to be, as the London Clay is a very soft material, easily eroded.

So, it was a fascinating walk, and I seem to have retained rather more of it than I thought I had at the time. I will certainly look at the woods in a new light!

And for those of you who read my piece on Crape Myrtle last week, I stopped to check out the bark on the tree and it is indeed both rather attractive and very smooth. What amused me no end is that having noticed one small tree, I completely failed to notice that there was another Crape Myrtle next to it. It just goes to show how distracted I’ve been, but no longer!

Crape Myrtle bark