Geology Along the Porter Brook

 

The Rough Rock downstream of the Hangingwater Road bridge

On the two previous occasions that I had walked along the Porter Brook, from Ringinglow and when exploring the area around Forge Dam with the Sheffield U3A Geology Group, I had seen various rock exposures that have been highlighted in the Porter Valley Geology leaflet by Peter Kennett of the Sheffield Area Geology Trust (SAGT). 

 

Flaggy Rough Rock exposed in the streambed of the Porter Brook

 

Between Hangingwater Road bridge and the Shepherd Wheel, the north-easterly dipping strata of the Rossendale Formation pass upwards from the Rough Rock sandstone to mudstones, a ganister at the level of the intermittent Pot Clay Coal seam and to the black shales of the Subcrenatum Marine Band – the designated boundary between the Upper Carboniferous Millstone Grit Group and the Coal Measures. 

 

Flaggy Rough Rock exposed in the streambed of the Porter Brook

When trying to get a good viewing point to photograph Hangingwater Road bridge, from the north side of the Porter Brook, I noticed that there were outcrops in the streambed and in the south bank that are very flaggy in nature. 

 

An exposure of flaggy Rough Rock

I returned to the bridge to take another record photograph and then just carry on downstream; however, having my Estwing hammer with me and plenty of time left in the day, I decided to go and further investigate the Porter Brook and see if I could obtain access to the exposures that I could see in the stream bank. 

 

A view downstream of Hangingwater Road bridge

 

Taking advantage of the low water level in the brook, I managed to cross over to the south bank – again jumping between the outcrops of flaggy sandstone in the streambed, as I had done only a few days earlier at Charlton Brook. 

 

An exposure of Rough Rock in the south bank of Porter Brook

I very quickly explored a section of the stream bank, where the Rough Rock is exposed. At its western, the lowest beds are highly laminated and very fine grained and pass upwards into more massive sandstone, where individual beds are occasionally more than 200 mm thick. 

 

An exposure of massive and laminated Rough Rock

I obtained three small samples of the Rough Rock from this outcrop. The specimen from the higher, more massive beds is a very fine grained pale grey/brown sandstone, with some orange iron staining in the body of the rock and very fine black iron bearing minerals scattered throughout. Within the body of this sandstone, laminations are clearly visible and, as seen on an exposed bedding plane, these contain a moderate amount of muscovite mica. 

 

A sample from the massive beds of the Rough Rock (21mm coin)

Of the two samples taken from the lower beds, one has a greater degree of lamination and higher silt content than the massive rock, but its colouration and mineralogy are quite similar. The second sample has the same general characteristics, but has internal laminations that are much darker in colour and particles of a much finer grain size. 

 

Samples from the laminated beds of the Rough Rock (21mm coin)