Common Common-weed

We stopped at Brimham Rocks (in Yorkshire, south of Ripon) for an ice-cream and a stroll about, and I was soon preoccupied by patches of this plant.

"I'm sure it's something really normal but for some reason my mind's gone blank," I said. 

"So when you look it up and find it's just Common Common-weed --" said Richard. 

"Yes, exactly," I said, trying to take an adequate photo, but not trying very hard.

Well, we turned out to be right, but I had excuses. 

It is Common Hemp-nettle (Galeopsis tetrahit, Sw: Pipdån), but firstly I all too rarely do any walking in acid habitats, and secondly it's seemingly quite a variable species.  These ones, with white flowers and deeply flushed calyces, looked very different to the ones I'd seen before, which have pink flowers and green calyces. 

And common as it is, there are some peculiarities about it: its occurrence in both arable land and in semi-shaded woodland like this -- not a frequent combination of habitats; its somewhat vexed relationship to Bifid Hemp-nettle (Galeopsis bifida, Sw: Toppdån); and its unexplained rarity in most of Ireland. 

All discussed in these articles:

https://fermanagh.bsbi.org/galeopsis-tetrahit-l

https://fermanagh.bsbi.org/galeopsis-bifida-boenn

A couple of the rocks. Carboniferous sandstone (Millstone Grit group).

The cross-bedding shown e.g. in the pic below reflects deposition in moving water.

I think this is what I. O. Evans is talking about in The Observer's Book Of Geology (revised edn, 1971):

"Such an arrangement of varied curves is called current-bedding, also false-bedding because it is attributed to currents in the water at the bottom of which the beds were formed; it may be compared to the series of oblique layers formed in a slag-heap as material is tipped over the sloping end of a horizontal mound."

Trunk of a Downy Birch (Betula pubescens) at Brimham Rocks. I always think.of this species as having rather dull bark compared to Silver Birch, but the younger trees can be very good-looking.