Aren't all the gritstones sedimentary, not aeolian? Or can they be both? I thought you only got shapes like that in arid regions where dry sand carried by the wind actually does the eroding, I don't think it's the actual moving air that does it.(i know dangerously little about geology though - last time I studied any was 1986. I'm sure that someone considerably more knowledgeable will now take great pleasure in telling me i am talking bollocks)
ShapingMost of the rocks owe their bizarre shapes to erosion during and after the Devensian glaciation. For example, Idol Rock was most likely formed just after the last glaciation when the land lacked any plant cover. Here, sand-blasting at ground level wore away the softer layers of the rock producing a tiny plinth with a massive top. Freeze-thaw action on the joints and bedding planes have shaped many of the tors such as the Dancing Bear.
JB's on the money. As for "palaeowinds" then I would hazard a guess that it's pretty much the same today - i.e prevailing sou'westerlies, palaeolatitude hasn't changed on a timescale relative to erosion and therefore I doubt weather systems would have changed much either.
Quote from: andi_e on February 26, 2009, 04:52:17 pmJB's on the money. As for "palaeowinds" then I would hazard a guess that it's pretty much the same today - i.e prevailing sou'westerlies, palaeolatitude hasn't changed on a timescale relative to erosion and therefore I doubt weather systems would have changed much either.Interesting - you've touched on part of the contreversy, whether they were formed by freeze thaw action - or by aeolian abrasion. Wind can erode without sand (dust abrasion) in a surpisingly effective way (esp around the lee of obstacles)... The palaeowind thing is whether or not during the early Holocene/end of last glaciation atmospheric circulation patterns were different (which they may - or may not have been..).Apparently the harder more westerly grits are largely deltaic, and the more easterly are predominantly dune deposits (e.g. Brimham) but thats what the sedimentologist in the office opposite said anyway! THeres a real dearth of this in the literatrue and many of the forms look distinctly aeoloian... Right, better finish teaching the 1st years....
Quote from: tomtom on February 26, 2009, 05:03:27 pmQuote from: andi_e on February 26, 2009, 04:52:17 pmJB's on the money. As for "palaeowinds" then I would hazard a guess that it's pretty much the same today - i.e prevailing sou'westerlies, palaeolatitude hasn't changed on a timescale relative to erosion and therefore I doubt weather systems would have changed much either.Interesting - you've touched on part of the contreversy, whether they were formed by freeze thaw action - or by aeolian abrasion. Wind can erode without sand (dust abrasion) in a surpisingly effective way (esp around the lee of obstacles)... The palaeowind thing is whether or not during the early Holocene/end of last glaciation atmospheric circulation patterns were different (which they may - or may not have been..).Apparently the harder more westerly grits are largely deltaic, and the more easterly are predominantly dune deposits (e.g. Brimham) but thats what the sedimentologist in the office opposite said anyway! THeres a real dearth of this in the literatrue and many of the forms look distinctly aeoloian... Right, better finish teaching the 1st years....Most grits in N yorks look too coarse grained and too poorly sorted to be aeolian - tough to explain those very common clasts of mudstone that weather out of breaks in most of the grits in north yorkshire without a fluvial origin.re the weathering, my money would be on freeze thaw/spalling of the originally less well cemented beds close to ground surface aided by interaction with more acidic groundwater/surface water , then occasionally exacerbated by wind erosion - just my thoughts though.
How about the Woolpacks?
Apparently the harder more westerly grits are largely deltaic, and the more easterly are predominantly dune deposits (e.g. Brimham) but thats what the sedimentologist in the office opposite said anyway! THeres a real dearth of this in the literatrue and many of the forms look distinctly aeoloian...
Quote from: stom on February 26, 2009, 09:40:25 pmHow about the Woolpacks?Ace, Where are they?
re the weathering, my money would be on freeze thaw/spalling of the originally less well cemented beds close to ground surface aided by interaction with more acidic groundwater/surface water , then occasionally exacerbated by wind erosion - just my thoughts though.
Hi All,Almost all the sedimentary rocks that make up the Millstone Grit successions across the Pennines were deposited by flowing water during the Serpukhovian (part of the Middle Carboniferous ~ 328-318 Ma). For the most part the gritstone that we know and love accumulated in a series of fluvial channels that flowed across a large delta plain whereby rivers drained southwards from the Caledonide Mountains to the margin of an enclosed sea that occupied much of the English Midlands at this time. Thus, these strata are principally fluvio-deltaic in origin. There are one or two exceptions to this, the most notable being Almscliff, which is considered (by some) to represent a submarine slide unit, whereby the leading edge of the delta failed and the entire unit moved down a marine slope.The differences in texture between, say, the Bridestones and Brimham reflect differences in the type of diagenetic (post-depositional) cement. The principal cement types are siliceous and calcitic, though their degree of development is variable. It is the extent of cement development that dictates how soft the rock is - soft bands erode more readily.None of the Carboniferous gritstone deposits of the Pennines are aeolian in origin (i.e. these sediments are not the deposits of windblown sand dunes in a desert). However, many outcrops have undergone considerable aeolain abrasion in relatively recent (Pleistocene-Holocene) times. Thus, the various pinnacles illustrated (e.g. the Bridestone) are relatively recent erosional aeolian landforms. The most prominent undercut bases arise where i) a bed of poorly cemented gritstone underlies a bed of better cemented gritstone and b/ where this occurs close to the ground where the effects of aeolian saltation (sand blasting) are most pronounced. Landforms such as the Bridestone and the Turban and Brimham would have likely developed most readily in early postglacial times at the end of the last ice age after the glaciers had retreated but before the sandy substrate had been stabilised by vegetation and peat development. This aeolian erosion is minimal in the current climate because loose sediment capable of aeolian abrasion is not available for transport via the wind.
