11 a.m.: In a sun trap of a back garden in South Ossett this holly blue is so intent on feeding on a flowering shrub that I’m able to get within macro range with my camera. When I see a blue butterfly the size of my little finger nail I’m never sure whether I’m looking at small blue, common blue or holly blue but, once it settles, the holly blue is the only one that that has bluish white underwings with small dark spots.
The small blue has black spots fringed in white on its pale grey underwing; the common blue has black and orange spots, also fringed in white, on a grey-brown background.
Along most of this stretch of the valley the River Calder is kept within its course by flood embankments except after exceptionally heavy rain, such as in last winter’s Boxing Day floods, but once it has overtopped these manmade levees there’s no direct way for the water to make its way back to the river as the flood subsides.
Increasingly, the canal, the Calder & Hebble Navigation, which runs parallel to the river, acts as an overflow channel but downstream from the Figure of Three locks the canal itself has what effectively acts as an embankment possibly built up in part by the navvies tipping spoil alongside the channel when they excavated this stretch of the navigation in the 1830s.
Following the Boxing Day floods, the farmer’s solution appears to have been to flatten a short stretch of the banking and to cut an overflow notch so that the field can drain across the towpath into the canal.
The excavations have exposed alluvial deposits which are typical of this stretch of the Calder valley: sandy silt containing pebbles of what appears to be local sandstone.
As I understand it, these deposits were laid down after the last ice age in a valley that had been deepened because the river that occupied it was heading down towards a lower sea level. Sea levels were so much lower during the ice age that most of the North Sea was dry land.
Swollen by meltwater, this precursor of the Calder was more powerful than the river as we see it today, which meanders over its flood plain re-sorting the alluvial deposits by cutting into the riverbank on the outside of a meander and depositing a sandbar elsewhere. I imagine that flash floods were more powerful in the treeless landscape of the ice age.
In the 18th and 19th centuries, stretches of the river were channeled into a straighter course by canal building navvies and much later in the early 1960s the river was diverted during the construction of Healey Mills railway marshalling yards.