Lowland Glaciation North Wales Background Although there have been many glaciations and advances in ice, the most significant for this are was the Dimlington Stadial which was a period of glacial advance during the Devensian epoch. The glaciers that originated in the Snowdonia mountains travelled northwards towards the Irish Sea. In the Ice Age, sea levels would have been much lower than they are at present, in fact, so low that Ireland was joined onto mainland Britain. The Snowdonia glaciers made their way out towards what is now the Irish Sea and deposited large amounts of boulder clay or till across the landscape. As the climate cooled even more, the Irish Sea Ice travelled south west covering the deposits dropped by the Welsh glaciers. They too smeared large quantities of glacial till over the landscape. This now overlies the Welsh glacial drift along the coastal plain. Periglacial head and soil Irish sea drift red sandstone matrix)
Welsh drift from the Ogwen Valley and Idwal Note dark grey matrix From slate and igneous Rock. The Irish Sea Ice not only deposited material, it also eroded the Isle of Anglesey, scouring out the rock to align the rock structure so that the landscape is aligned in a north-east to south-west direction. On the mainland this same direction of flow can be seen in the drumlinoid features lying on the coastal plain between the Snowdonian mountains and the Menai Straits. AberOgwen Features to be seen: Varves This area would have been an outwash plain for the Snowdonian ice. Meltwater streams would have run across the landscape, transporting smaller glacial deposits away from the snout of the glacier. In places there will have been depressions. These may have started life as kettle holes (depressions where lumps of stagnating ice have melted ) which have then become proglacial lakes.
The proglacial lakes will have been in coexistince alongside the glaciers. In the warmer months of spring and summer the amount of melting will have increased. This meant that the meltwater streams had more energy and were able to transport larger particles such as sands and gravels away from the glacier. When the streams flowed into the lake there would have been an immediate loss in momentum, velocity and therefore energy. The stream then deposited the load it carried. In the autumn there will have been less water from melting and so the streams will not have been able to transport as much load. Hence with less energy, the material carried was smaller. Again on reaching the lake this material was deposited. Into late autumn and winter the lightest and finest sediments finally settled on the lake bed as the stationary water had no energy. This left a layer on very fine clay particles. As a result a series of layers built up; coarser, sandy deposits followed by finer, clay deposits.
In places these clay deposits are folded and contorted sue to a process known as cryoturbation. This occurs in periglacial environments. Periglacial locations are usually typified by permafrost or permanently frozen ground. Above this is a layer of ground which thaws each summer and freezes in the winter months. As that refreezing occurs, the freezing moves from two directions, from above as air temperatures freeze the ground surface, and from below as the permafrost advances upwards. The soft ground imbetween is squeezed like a tube of toothpaste resulting in a stirring up of the layers of clay.
By studying the deposits and the sequence of strata a picture of the formation of this landscape can be built up. Originally the Welsh ice extended across the coastal plain. The ice retreated in a warmer period leaving an outwash plain or sandur. This would have been an area of sands, gravels and pebbles, rounded by fluvioglacial attrition. This area may have had stagnant boulders of ice. As these melted, they left depressions in the gravels. These are known as kettleholes. These would have filled with water. Meltwater streams flowing into these deposited their sediments as layers seasonally. As the lake filled in, meltwater streams continued to flow across the landscape, leaving gravel and pebble cross-bedding from the slip-off slopes of meanders. Boulder clay
The Welsh boulder clay at Aberogwen. Note the unsorted nature of the Material and the variety in angularity. Pentir Pentir has many lowland glacial features. The area around Pentir is comprised of drumlinoid field all aligned in a north-east, south-west direction. Kames and eskers and kettleholes can all be found in the vicinity. The Pentir Esker An esker is a subglacial stream bed. It is formed by the build up of fluvial deposits under the glacier. Morphology: eskers are linear in shape, ie long and thin and sinuous They meander like streams because they were formed by subglacial streams. The Pentir Esker is around four hundred metres in length and around five metres high. Composition: coarse sands and gravels.
The Pentir esker seen from the side Table : the Pentir Esker Characteristics Details Formation Formed by subglacial streams usually in time of glacial retreat. The channel is contained within a tunnel of ice. Stream sediments build up the floor of the channel as there is no flood Morphology plain. Over time the channel floor is raised above the land on either side. Composition Long, thin and sinuous ie meanders Around 400m in length, and 5metres in height. Sands and gravels The Pentir esker seen from on top. The Pentir Kame
A kame is often formed where moraines have fallen into flooded crevasses. A certain amount of sorting of the materials will take place leaving stratified layers of sediments. In shape they are far less regular in form than drumlins or eskers. Note the bedding moving from top left diagonally down to bottom right. This will have probably occurred when the creavasse collapsed causing the stratified material to slump. (photograph: Helen Morton) Stones from the kame deposit. Note the roundness, many are subrounded or rounded on the Powers Index. (photograph: Helen Morton)
Kettleholes Several of these can be found in the low-lying coastal plains around the area of Pentir and Caernarfon. Very few are still water-filled because over time they have filled in. However, there are many marshy and boggy hollows where hydroseres have developed. Most of these are filled with water-loving trees such as willows. They have not been used for agriculture as they are too wet. There is one such example near the kame at Pentir. It is a low hollow, around 75 metres in diameter, filled with hydrophytic (water-loving) plants such as the yellow iris and willow tree. Table 1: Fluvioglacial and glacial deposits compared Angularity Glacial drift Angular and subangular if deposited as boulder clay (ground down due it abrasion) Fluvioglacial deposits Rounded sediment due to attrition in meltwater streams. Size Mixture of all different sizes of material. Completely unsorted. Sorted material where material is found alongside material of the same size. Often these are stratified into layers.
Orientation Usually aligned in the same direction of flow as the glaciers. As a result all the deposits are strongly aligned in one direction. Although water also aligns material in the direction of flow, the meltwater streams are much smaller and also meander over the outwash plain. Meltwater streams often change course as levels rise and fall, leaving a series of river channels and islands. This is known as braiding.