As readers of this blog will know, I have frequently bemoaned the fact that we know a lot about ice sheet collapse and disintegration, but not a lot about the manner in which ice sheets grow, through a process which we might call "blanketing the thickening". See the following:
From this earlier post:
Following a warm (interglacial or interstadial) episode…. the first thing that happens is that temperatures drop, with the mixed deciduous forest replaced by a boreal forest and then by tundra. Then we see the growth of seasonal and then continuous permafrost. That creates conditions where seasonal snow-melt is reduced and eventually eliminated. So where snowfall is substantial (fed by moisture-bearing winds) snowfields develop in the highlands and then in the lowlands, with seasonal melting having less and less of an effect until the whole landscape is covered with snowfields and firn. In favourable locations — on the highest plateaux and in mountainous areas where there are substantial snow-collecting hollows facing north or north-east the firn might thicken substantially until we see the creation of glacier ice. Where there are convenient slopes or discharge routes, glaciers may start to flow and expand, pushing into areas which are otherwise covered with perennial snowfields.
But at what point does streaming ice from far afield start to affect an area like West Wales, where there may be lowlands, upstanding hill masses and plateaux which might support local ice caps? It's clear that this happens very late in the day, towards the back end of a full glacial cycle. The glacierization build-up phase (with extensive snow and ice cover on the landscape) may last for 70,000 years or more, interrupted by short-lived cooling and warming phases or by changes in precipitation and other types of "climatic forcing". But eventually, when the whole land surface has become deeply buried by locally-generated ice, maybe 500m or more in thickness, the big brute from the north begins to dominate -- in this case with the arrival of the Irish Sea Ice Stream - as cold-based ice is replaced by ice that is capable of sliding on its bed. Streaming ice then begins to do serious damage, eroding bedrock, picking up erratics and incorporating pre-existing periglacial and other deposits, and modifying a previously protected landscape in a multitude of different ways. The direction of ice flow is now determined not by the details of local topography but by the surface gradient of the ice stream; in the case of Pembrokeshire ice movement seems to have swung through an arc of maybe 60 degrees, but with a dominant flow from NW towards SE. It appears that the Welsh ice cap did not greatly affect Pembrokeshire, but ice from the Welsh uplands may well have filled Cardigan Bay, creating a constriction that affected Irish Sea ice flow directions. From the BRITICE-CHRONO modelling, Irish Sea ice affects the area for maybe 3,000 years.
This is an interesting short article:
It makes the point that the ice in these small ice caps and ice fields (which were probably initiated in the Neoglacial, after c 7,000 yrs BP) is very thin and cold-based, frozen to its bed, and effectively preserving without disturbance what was there before -- both landforms and sediments.
In previous posts I have speculated on the lessons that might be learned from Drangajokull and Glamajokull in NW Iceland. Use the search facility to look them up!
As I have suggested on earlier occasions, Salisbury Plain may well have been submerged beneath an extensive snowfield / icefield for many thousands of years, over and again during the Quaternary. The enigmatic clay-with-flints may tell us something about what happened, and when........ But if the bluestones at Stonehenge really are glacial erratics, carried in by the ice from the west (and maybe the north as well), exactly how did this moving ice arrive, and what were the basal ice conditions at the time? It may well be impossible to determine where the ice edge was located, because that would have been impossible if we had looked down at a white surface from a satellite at the time. More to the point, where was the easternmost extent of MOVING ice capable of carrying extraneous or erratic material? Work in progress.......
"Kama wasomaji wa blogi hii watajua, mara nyingi nimesikitika ukweli kwamba tunajua mengi juu ya kuanguka na kuharibika kwa barafu, lakini sio mengi juu ya jinsi barafu inavyoongezeka, kupitia mchakato ambao tunaweza kuiita ""kufunika unene.""" Angalia ifuatayo: Kutoka chapisho hili la awali: Kufuatia kipindi cha joto (kati ya barafu au kati ya hatua).... Wakati wa joto, misitu ya mchanganyiko ya majani huchukuliwa na misitu ya boreal na tundra. Kisha tunaona ukuzi wa barafu ya kila msimu na kisha ya daima. Hilo hufanyiza hali ambapo kuyeyuka kwa theluji kwa majira hupunguzwa na hatimaye kuondolewa. Kwa hiyo, mahali ambapo theluji inanyesha sana (kwa sababu ya upepo wenye unyevunyevu), mashamba ya theluji hutokea katika maeneo ya juu na kisha katika maeneo ya chini, na kuyeyuka kwa theluji kwa majira kunakuwa na athari ndogo na ndogo hadi eneo lote likiwa limefunikwa na mashamba ya theluji na firn. Katika maeneo mazuri - kwenye nyanda za juu zaidi na katika maeneo ya milima ambapo kuna mashimo makubwa ya kukusanya theluji kuelekea kaskazini au kaskazini mashariki - firn inaweza kuwa nzito sana hadi tuone uundaji wa barafu ya barafu. Mahali ambapo kuna mteremko mzuri au njia za kutiririka, barafu zinaweza kuanza kutiririka na kupanuka, zikisukuma katika maeneo ambayo vinginevyo yamefunikwa na maeneo ya theluji ya kudumu. Lakini ni wakati gani barafu inayotiririka kutoka mbali huanza kuathiri eneo kama Wales Magharibi, ambapo kuna maeneo ya chini, milima ya juu na nyanda ambazo zinaweza kuunga mkono mipaka ya barafu ya ndani? Ni wazi kwamba hii hutokea mwishoni mwa siku, kuelekea mwisho wa mzunguko kamili wa barafu. Kipindi cha kuunda barafu (kwa theluji na barafu kubwa kwenye mandhari) inaweza kudumu kwa miaka 70,000 au zaidi, ikivunjwa na vipindi vya baridi na joto vya muda mfupi au kwa mabadiliko ya mvua na aina nyingine za "kulazimisha hali ya hewa." Lakini hatimaye, wakati uso wote wa nchi kavu umezikwa na barafu ya ndani, labda mita 500 au zaidi kwa unene, mnyama mkubwa kutoka kaskazini huanza kutawala - katika kesi hii na kuwasili kwa Mtiririko wa Barafu ya Bahari ya Ireland - kama barafu ya baridi inabadilishwa na barafu ambayo ina uwezo wa kuteleza kwenye kitanda chake. Kisha, barafu inayozunguka-zunguka huanza kusababisha uharibifu mkubwa, ikiharibu miamba, ikichukua miamba isiyo ya kawaida na kuingiza mabaki ya barafu na mengine yaliyopo hapo awali, na kubadilisha mazingira yaliyolindwa hapo awali kwa njia nyingi tofauti. Mwelekeo wa mtiririko wa barafu sasa huamuliwa sio na maelezo ya topografia ya eneo hilo, lakini na gradient ya uso wa mtiririko wa barafu; katika kesi ya Pembrokeshire, mwendo wa barafu unaonekana kuwa umepiga kupitia arc ya labda digrii 60, lakini na mtiririko mkubwa kutoka kaskazini-magharibi kuelekea kusini-mashariki. Inaonekana kwamba barafu ya Wales haikuathiri Pembrokeshire sana, lakini barafu kutoka maeneo ya juu ya Wales inaweza kuwa imejaa Ghuba ya Cardigan, na kuunda mkondo ambao uliathiri mwelekeo wa mtiririko wa barafu ya Bahari ya Ireland. Kulingana na utafiti wa BRITISE-CHRONO, barafu ya Bahari ya Ireland inaathiri eneo hilo kwa miaka 3000. "Hii ni makala fupi ya kuvutia: ""Inaonyesha kwamba barafu katika hizi ndogo ice caps na mashamba ya barafu (ambayo pengine ilianzishwa katika Neoglacial, baada ya c 7,000 BP) ni nyembamba sana na baridi-msingi, kufungia kwa kitanda chake, na kwa ufanisi kuhifadhi bila usumbufu kile kilichokuwa hapo kabla - wote landforms na sediments.""" Katika makala zilizopita nimekuwa kufikiri juu ya masomo ambayo inaweza kujifunza kutoka Drangajokull na Glamajokull katika Northwest Iceland. Tumia kituo cha utafutaji ili kuwatafuta! Kama nilivyosema katika matukio ya awali, Salisbury Plain inaweza kuwa imezama chini ya barafu kubwa kwa maelfu ya miaka, mara kwa mara wakati wa Quaternary. Huenda udongo wenye mawe ya kuogelea ulituambia jambo fulani kuhusu kilichotokea, na ni wakati gani........ Lakini ikiwa mawe ya bluu huko Stonehenge ni ya barafu, yaliyopelekwa na barafu kutoka magharibi (na labda kaskazini pia), basi barafu hii ya kusonga iliwasilije, na hali za barafu zilikuwaje wakati huo? "Hata hivyo, ni vigumu kujua mahali ambapo barafu ilikuwa, kwa sababu hiyo ingekuwa vigumu kama tungekuwa tumetazama chini kwenye uso mweupe kutoka kwa satelaiti wakati huo.""" Zaidi ya uhakika, ambapo ilikuwa mashariki zaidi kiasi cha kusonga barafu uwezo wa kubeba nyenzo ya kigeni au erratic? Kazi inayoendelea.......