Evidence for ice

This section examines features created by ice in places where the action of  ice is still current or relatively recent, it then looks for similar evidence in western Herefordshire, beginning with stones and till.

Linear moraines and lakes
Meltwater channels
Hummocky moraines
Kettle holes and ponds

Stones and till

AZ - GlacierBasal

Stones and till
The photographs show the basal part of a modern glacier in Switzerland with layers of ice richer and poorer in rock debris.

The glacier erodes underlying rock and the particles rub against each other in the ice. The effects are to give larger rock fragments distinctive shapes and to create a lot of fine muddy rock matrix.

AZ - UnderGlacier

View under a glacier
View underneath a glacier at Marmolada, Italy, showing the basal ice with debris on top of bedrock.

AZ - StriatedStones

Glacially transported stones
Glacially transported stones from a modern glacier in Canada (inset) and in Mowley Wood, Arrow valley, Herefordshire.

Transport under the ice leads to new flat faces by breakage of stone followed by rounding of corners and scratches caused by rubbing of fine rock particles.

AZ - Till

Till is a deposit laid down in association with a glacier and consists of a mixture of all grain sizes: gravel, sand, silt and clay as they are released from ice by melting.

Left photo is subglacial till from a modern Swiss glacier, Tsanflleuron.
Right photo till in western Herefordshire, Moccas.

AZ - AblationTill

Ablation till
Till may be plastered on the substrate underneath a glacier (subglacial till) or form by slow melting of ice (ablation till).
Photo shows ablation till forming from debris-rich ice that has been thrust onto the surface of the Saskatchewan glacier, Canada.

Linear moraines and lakes | back

AZ - Moraine

Linear moraines and lakes
Linear moraines form parallel to the ice flow or form arcs at its margin.

This arcuate example marks the terminal moraine of the Tsanfleuron glacier, Switzerland in the years 1855-1860.
The moraine impounds lakes and is cut by a meltwater stream draining the modern glacier.

AZ - Moraine2

Recessional moraines
View towards the Tsanfleuron glacier. Small parallel mounds in the foreground mark minor annual re-advances during the retreat of the ice margin. These are called recessional moraines.

In Herefordshire, there is no terminal moraine and the largest moraines such as the Staunton and Orleton moraines are recessional. They represent periods of stand-still of probably decades or centuries.

Hummocky moraines | back

AZ - IceCoreMoraine

Hummocky moraines
At the margin of the glacier, the rate of melting equals the supply of ice from glacier flow. These are often complex, chaotic places where sediment, water and ice are intimately mixed. When a glacier is in long-term retreat, a significant part near the margin may stop moving. This stagnant ice gradually melts.

The photo is a moraine in Iceland.

AZ - ChaoticMoraine

Hummocky moraines
Moraines at a chaotic ice margin, Switzerland.

AZ - StagnantIce

Hummocky moraine
Hummocky moraines just beyond the limit of the Saskatchewan glacier, Canada, and underlain by stagnant ice. Over time, the topography can “invert” and mounds cored by clean (sediment-free) ice become depressions whilst sediment-filled depressions end up as mounds.

The hummocky terrain in western Herefordshire vary considerably in the size, shape and orientation of mounds and their origin is the subject of ongoing research.

In this account, we emphasize the development of hummocky moraines by meltout of stagnant ice.

There are several other mechanisms for formation of mounds in glacial terrains and other terms (e.g. kames, drumlins) are used. For example, material may be thrust to the surface of the ice (leading to a type of hummocky moraine where they occur in lines), or short-lived rivers can build out deltas (a type of kame), or glacial erosion can shape the substrate into a smooth elongated form (drumlin).

The hummocky terrain in western Herefordshire vary considerably in the size, shape and orientation of mounds and their origin is the subject of ongoing research.

Meltwater channels | back

AZ - Channel1

Meltwater channels
Meltwater forms very energetic flows, starting from small streams flowing over the ice surface as in this example from Svalbard. A false step on the icy bed of the stream could lead you to be swept away down a crevasse to the bottom of the glacier!

AZ - Upwelling

Melwater channels
Dramatic evidence of the upward flow of pressurized subglacial water is shown by this discharge from below into the Sula river, Iceland.

AZ - Turbulent

Meltwater channels
Erosion by turbulent fast-flowing glacial meltwater occurs on all scales and is particularly effective where the water is carrying sediment. Here is a small meltwater erosional channel 0.5 m across in bedrock, Garvellach Islands, Scotland.

AZ - Channel3

Meltwater channels
An origin as a meltwater channel in areas that have been glaciated is suggested by valleys that carry no water or only a small (misfit) stream as in this example from Killegar, Ireland. The valley in the skyline is the channel.

AZ - Channel4

Meltwater channels
Under the field in the photo above is a channel sediment shown here to be a boulder conglomerate. Such partly sediment-filled channels, called tunnel valleys, separate the deposits of successive ice ages in deposits under the North Sea.

Photo taken in 1996. A modern risk assessment would forbid this geologist from approaching this dangerous face.

AZ - Channel5

Meltwater channels
Excellent evidence for the existence of subglacial water under pressure during Herefordshire glaciation is provided by a relic channel known as “the Rainbow”, west of Kington. The channel branches off from the modern stream at point X and rejoins at point Y. The channel bed rises, then falls, with distance (X to Y – also see diagram below), unlike the modern River Arrow which of course falls continuously between these same points.

AZ - Channel6

Meltwater channels
The purple line is, the height of the ancient meltwater channel with distance.

The graph clearly shows a section where the channel goes uphill. This can only be explained by water under significant pressure confined beneath the ice.

AZ - Hubbard1

Metlwater channels
One particularly effective means of promoting meltwater erosion is where there is an outburst flood from a glacial lake. A modern example is shown in photographs from Hubbard Glacier, Alaska where the glacier had squeezed out a meltwater stream against a cliff. This caused the damming of a temporary lake whose water level rose by 19 metres over a period of weeks before the natural dam to the lake burst and was eroded away.

AZ - Hubbard2

Meltwater channel
The water level raised 19 metres.

AZ - icelake

Meltwater channels
In western Herefordshire there are several examples of large glacial lakes which overflowed and led to the erosion of new river courses in gorges (see part 3 of the Car/Cycle Tour). The Herefordshire gorges do not support vertical sides because the bedrock is too soft.

Kettle Holes and Ponds | back

AZ - kettlehole

Kettle Holes and Ponds
We refer to ice age ponds as those whose origins can be traced back to the ice age. Many depressions in the landscape were created by erosion during the ice age. However a particularly distinctive feature are kettle hole ponds, which originate, normally in hummocky moraine terrain, by melting of ice.

If a pond shows the following characteristics, we identify it as a kettle hole:
1. Forms a closed depression in the landscape (elongate forms are likely to be channels not kettle holes)
2. Surrounded by glacial sediments
3. Where evidence is available from geophysics or coring, the pond is underlain by sediments different from the surroundings.

AZ - kettleholeFormation

Kettle Holes and Ponds
Cartoon to show the formation of a kettle hole pond and its filling by peat or other deposits. There comes a point when the glacier stagnates and the ice stops moving forward. Then this “dead” ice gradually melts away and locations where there is little sediment end up as depressions, although this process can take thousands of years to be completed.

Although peat is shown as their infill, there is often just as much washed-in sediment too. The oldest infill in Herefordshire kettle holes found by carbon- dating formed around 13,500 to 15,000 years ago.

AZ - Site15TourMap

Kettle Holes and Ponds
These images, from Site 15 on the Car/Cycle Tour2 , map out the catchments of the ponds to show how they lie in closed depressions even in a low-relief area.

The top image is produced using LIDAR, the lower image conventional air borne photography. Light blue = ponds mapped from satellite images. Dark blue = ponds mapped in the field.

The top image shows a typical network of depressions within an area of hummocky moraine.

AZ - MoccasGeol

Kettle Holes and Ponds
Another origin for ice age ponds is as fillings of channels cut by meltwater. The peat-filled Rainbow channel is a good example. Another possibility is a chain of ponds several kilometres long near the south margin of the glacier, part of which is shown in this image of the Moccas Park area. The depressions are filled by peat and separated by till.