You can drive quite close on a dirt track on the edge of the woodland and take a path in. It's quite near a 'Poney-Club'. I don't remember a signpost but this grid ref is correct.

It lies in the thick woodland of the Foret Domenial L'Isle Adam, and is a sunken length of megalithic drystone walling between orthostats, still bearing the weight of four capstones. At the portal end, a carved stone has a kennel hole in it which presumably once had a blocking stone. All round the allee couverte, which is fenced for protection, wild strawberry plants formed a carpet underfoot. A very nice monument.

Just a few fields away from La Pierre Plate is the allee couverte de Blanc Val, which has lost all its capstones and now lies in a scrubby patch of thyme and weeds at the edge of a field near Presles.

Blanc Val has a complete set of internal orthostats lining what is essentially a stone box. At one end, the bottom half of the portal stone is still visible, with the semicircle of what was once a porthole entrance. Blanc Val is not a thrilling monument to see, but the fact that it's still here and that traces of cement on it shows that someone recently gave a toss about it is good enough for me.

La Pierre Turquaise is a big league A-list 'Hollywood' megalithic site! Nestled into mature oak forest, and with acorns raining down all around us, we sat drinking tea from our flasks and smoking a small cigar to help us relax and admire this lovely place. In 1842 it was nearly broken up to help pave Paris, would you believe and in 1985 it suffered a horrific explosive attack which badly damaged it. One of the stones was sheared off and the top piece lies near the allee couverte.

It's 10.6 ms long and nearly 2 ms wide and despite the 1985 attack still has 2 mighty capstones. Its calm woodland setting reminded us of some of the hunebedden in NL. Hard hats required though, to protect yourself from all those falling acorns. That, or bring a piglet!

We set out early to find the allee couverte du Bois-Couturier near Guiry-en-Vexin. They have maps to show you how to find it at the prehistory museum in Guiry. You have to walk about 20 minutes through a forest, on reasonably good clearly marked paths, to find it. But when you have you'll love it!

It was rediscovered in 1915 and excavated until 1919: 200 skeletons found as well as polished stone axes, tools and ceramic shards. It's 8 ms long and 2 ms wide – I thought the proportions seemed quite odd compared with many allee couvertes we'd seen. Essentially it's a sunken box entirely lined with drystone walling and covered with 4 capstones.

The most astonishing thing about it is the round and lovely porthole in the portal stone – quite perfect! I crawled through it and into the tomb. I sat inside to consider that that the round blocking stone 'door' was discovered during excavation, and in superb condition. It now lives in the nearby prehistory museum. It looks like a great saucepan lid, complete with handle.

This is a 'Hollywood' site by any standards.

It's hard to know whether to laugh or cry when you're faced with it! We'd seen a picture of it printed on the label of a bottle of beer the night before and been mightily amused. The reality of it is that it's quite small – about 5'3". Though how much the silly christians removed from its original size to fashion it into this bizarre cross is impossible to tell.