Ice sheets and frontal ice lobes

This map shows just how complex the LGM ice edges were to the south and east of the Baltic coasts -- there were lobes and embayments everywhere.  The article mentioned below relates to the area to the south of the Gulf of Riga.  The line marked "15" shows the approx ice edge 15,000 years ago.

I have been thinking about the behaviour of the front edges of ice sheets during advance, stagnation / stability and then deglaciation phases. This is of considerable importance to our understanding of what happened near the edge of the British and Irish Ice Sheet (or Celtic Ice Sheet) at the glacial maxima of however many glaciations there might have been.  How did the ice edge behave in the Bristol Channel area and in the Celtic Sea?  Did it actually reach Salisbury Plain, and if so, what was the shape of the ice front?  Was it a lobe or a straight ice edge?  (I have always argued for an Anglian Glaciation lobe of ice from the Welsh Ice Cap and from the Irish Sea Glacier, pushing into the Somerset Lowlands and bounded by hill masses to north and south.....)

A great deal is now known of the lobes at the southern extremity of the Laurentide Ice Sheet in North America -- more on that in due course.  But in the meantime, I have come across an interesting article on the Devensian / Weichselian lobe which extended southwards from the Scandinavian Ice Sheet around 15,000 years ago -- in the midst of multiple retreat stages and readvances over many thousands of years.

Reference:
Lamsters, K., Zelcs, V., Subglacial bedforms of the Zemgale Ice Lobe, south-eastern Baltic, Quaternary International (2014), 13 pp.

http://dx.doi.org/10.1016/j.quaint.2014.10.006

The topographic context -- the Zemgale Ice Lobe pressed into the lowland to the south of the Gulf of Riga, bounded and constrained by the uplands shown here quite clearly.

Ab s t r a c t

Presented in this paper are the results of the mapping of ~6600 subglacial bedforms of the Zemgale IceLobe (ZIL), their composition and internal structure in the south-eastern Baltic region. Topographic maps at scale 1:10,000 were mainly used to map bedforms in Latvia, while a digital elevation model with cell size of 5 m was used for North Central Lithuania. The ZIL operated during the deglaciation of the Late Weichselian Scandinavian Ice Sheet (SIS), at least in the Middle Lithuanian and North Lithuanian glacial phases, and created a subglacial landform assemblage consisting of glacial lineations such as drumlins, mega-flutings, Mega Scale Glacial Lineations (MSGLs) and transversal morphologies, such as ribbed moraines. It is observed that lineations of different scales co-exist, suggesting subglacial bedform transition and possibly a continuum of bedforms. Highly elongated lineations with a length of up to 24 km and an elongation ratio of up to 1:50, interpreted as MSGLs, are observed in the NE part of the Middle Lithuanian Plain. In the so-called Zemgale drumlin field, 20% of lineations have elongation ratios >10:1, indicating the fastest ice flow in the central part of the main body of the lobe. Based on test surveys and investigated outcrops, the drumlin cores consist of sorted sediments with different levels and depths of glaciotectonic deformation overlain by subglacial till. The main drumlin-forming ice stress was mostly parallel to the crests of drumlins, while some ice stress from inter-drumlin depressions acted during the final episodes of their formation. The observed sediment structures indicate multiple episodes of the basal ice/bed coupling and decoupling, which can best be explained by the mosaic ice-bed deformation model. Ribbed moraines are characterized by a complicated structure of multiple till units interbedded with sorted sediments that have been formed by repeated subglacial overthrusting.

https://www.academia.edu/9256806/Subglacial_bedforms_of_the_Zemgale_Ice_Lobe_south-eastern_Baltic?email_work_card=thumbnail

Drumins and other bedforms showing ice flow directions around 15,000 years ago.  Most of the observed "glacial lineations" are drumlins or elongated flutes, but here and there the patterns are confusing because Rogen moraines and other transverse ridges have also been formed in response to changing bed conditions.

The interesting thing about the Gulbene Glacial Phase is that the ice has responded in a very subtle fashion to topographic controls (uplands on 100m height were quite sufficient to divert ice which was quite thin). Also, by and large, the streamlined bedforms are perpendicular to the ice edge, as they should be.  In the later Linkuva Glacial Phase, the lobe was of more limited extent.  What is also intriguing is that the ice has swung round through about 160 degrees in the Vadakste area, almost flowing in the opposite direction to the original flow across the Baltic, from north to south.

All other things being equal, ice will fill depressions and spread laterally in response to quite subtle topographic controls.  I am more convinced than ever that when the Irish Sea Glacier was at or near its southernmost position during the LGM, most of the Bristol Channel and the outer reaches of the Severn estuary MUST have been covered with glacier ice.  And during the Anglian Glaciation, when ice was almost certainly more extensive, was there glacier ice on the Somerset Levels, at least as far east as the chalk escarpment?  Almost certainly.

If Anglian ice reached the shelf edge in the Celtic Sea, it is inevitable that the Somerset Lowlands were inundated by glacier ice.

Suggested Devensian (LGM) situation, with Celtic Sea ice pressing eastwards at least as far east as Gower, and morec restricted ice lobes in South Wales.

BRITICE 2016 reconstruction of the extent of ice at the LGM in the Celtic Sea arena.  I do not believe that the streamlines shown are feasible.  Ice MUST have reached the cliff ramparts of Devon and Cornwall, and MUST have extended further up the Bristol Channel.



Lobes and separating hilly ridges on the edge of the ice sheet in West Greenland.  Nature abhors straight lines, and glaciers love lobes.........