Processes leading to loss of snow and ice from a glacier, in our case primarily by melting.
The most intense glaciation in the UK, occurring around 450 thousand years ago, during which ice covered the whole of Herefordshire. In western Herefordshire, the glacial landscape is younger (Devensian), although there may be some erosional features and buried sediments that date from that time.
A device with a screw or open cylinder to penetrate the soil and underlying superficial deposits and obtain samples.
Hardened (cemented or lithified) rocks more than 2 million years old. In our area of interest these are predominantly mudstones and sandstones of Silurian and Devonian age (roughly 400million years old)– also known as Old Red Sandstone. Shown on geological maps unless any overlying superficial deposits are more than two metres thick.
A useful, but outmoded term, replaced in modern literature by diamict (see till) to describe a sediment made of all sizes of grains from mud to boulders (see gravel).
British Geological Survey
A public organization whose responsibilities include creating and maintaining geological archives such as maps.
BGS: Maps can be consulted using an app called iGeology (free to download) or on their website. Paper maps may be purchased via this website or from the map shop in Upton on Severn. They are convenient to use, but in some cases will be less up-to-date than the on-line resources.
Our area of interest is covered by map sheets 181 (Ludlow) 198 (Hereford) and 197 (Hay-on-Wye), but also in the area of 180 (Knighton) which is not published. There is an in-depth report (memoir) for sheet 198 and a brief report for sheet 197.
The last ice age which reached a climax around 25 to 20 thousand years ago, following a long period when glaciers were confined to the mountains. The Ice Age landscapes discussed in the IceAgePonds app and this website formed in the Devensian.
The yellow area depicts the location of Herefordshire.
A stone transported by ice from its original location. If the rock type can be matched with its source location, the distribution of erratics can demonstrate the flow direction of ice. In Herefordshire, a famous example is the gabbro rock of Hanter Hill, west of Kington, erratics of which form a trail stretching eastwards.
Sediment laid down in a glacial setting, including till, sands and gravels and muds.
A time period when glaciers are present on the land.
A body of ice moving downhill under its own weight.
Sediment made predominantly of grains larger than 2 millimetres. In our area, gravels were laid down by glacial streams. Gravel can be subdivided into granules (2-4 mm), pebbles (4-64 mm), cobbles (64-256 mm) and boulders (>256 mm).
A term used on geological maps to describe sediments that have moved downslope, typically slowly as a mixture of sediment, water and ice, in periglacial conditions. Head often contains a mixture of all sizes of grains, like till.
Our current interglacial. Refers also to sediments accumulated in the last 11,700 years.
Term used to cover a wide variety of different types of moraine that form distinct mounds rather existing as a line.
Hummocks may form by some combination of:
1) by thrusting in front of or underneath a glacier,
2) erosion by ice,
3) ablation of ice containing varying amounts of debris from place to place,
4) build-up of minor deltas.
Here we refer informally to Ice Ages as intervals during the last half-million years when glaciers advanced across parts of the UK. These are also known as glacials, separated by warmer periods, such as today, which are known as interglacials.
Ice Age Ponds
Ponds that occupy depressions created during glaciation. Examples are kettle holes and channel fillings.
Kettle hole pond
A pond that occupies a depression created by melting of ice. For Herefordshire examples, this ice is thought to be stagnant glacier ice, but more generally could also be icebergs grounded in a lake or ice blocks transported onto a floodplain after an outburst flood.
An acronym of light detection and ranging. It is a method of aerially surveying topography by firing laser pulses from an aircraft. The grid of height measurements can be transformed into hillshaded images and digital terrain models.
An erosional furrow cut by a stream of meltwater, varying in scale from less than a metre across to whole valleys. Meltwater channels may cut through superficial deposits to underlying bedrock.
The example below can be seen on the Breinton Walk.
Ridges or mounds formed of glacial sediment. Ridges may be parallel to or perpendicular to glacier flow, whilst mounds may be isolated or in groups, elongated or lacking elongation. Mounded moraine is referred to as hummocky moraine. A terminal moraine forms an arc at the furthest extent of the ice, but in western Herefordshire the ice limit is not marked by a moraine. Instead, the most conspicuous moraines parallel to ice margin were formed during retreat as recessional moraines, marking a still-stand of the ice margin (e.g. Staunton moraine, Orleton moraine).
Sediment made of particles finer than 1/16 mm. This can be divided into silt (1/256-1/16 mm) and clay (less than 1/256 mm). This can be divided into silt (1/256-1/16 mm) and clay (less than 1/256 mm). Mud is created chemically and biologically by weathering of rocks in soils, but also mechanically in the basal part of glaciers by crushing and grinding of rock particles.
A rock outcrop that stands up above a glacier.
Glacial streams that extend beyond the margin of a glacier. They typically have a braided pattern (see photo below) with many channels which wander across the floodplain over time, depositing sand and gravel.
An organic-rich sediment accumulating under swamp conditions of incomplete decay of organic matter. Peats can be found under kettle-hole ponds or other stagnant water bodies. Pollen recovered from peat allow a reconstruction of past vegetation.
An environment subject to repeated freezing and thawing. May be associated with glaciers, but in our area, permafrost conditions (as in the modern tundra environment) persisted for thousands of years after glaciation.
Permanently frozen ground, typically persisting metres or tens of metres below the ground surface. Normally there is a metre-thick active layer that melts each summer and, if on a slope, tends to flow downhill (see head).
The time period of the ice ages as seen in the northern hemisphere. Refers also to deposits formed between 2.6 million and 11,700 years ago.
Sediment composed of grains between 1/16 and 2 mm in size. In our area of interest, sand is typically associated with gravel and was laid down by glacial streams.
A mixture of weathered rock and organic matter forming at the ground surface in association with vegetation. Soils progressively alter the underlying bedrock and superficial deposits which are shown on geological maps. (See auger).
Refers to the region near the base of a glacier where sediment-rich ice occurs, meltwater may be under pressure, and rocks are crushed and ground.
Surface sediments that rest on bedrock. Where they are more than two metres thick, they are shown on geological maps. These deposits were formed during the Quaternary Period (seealso Pleistocene and Holocene) which started 2.6 million years ago.
A deposit laid down in association with a glacier. Roughly equivalent to the old descriptive term boulder clay (the modern term is diamict). However, diamicts can also form by downslope sediment movement. Tills can be laid down under the ice and, if so, are typically
1) well-consolidated under the pressure of the glacier,
2) sometimes sheared, and
3) pebbles become aligned parallel to ice movement.
Till can also form by ablation (melting and sublimation) of ice on its surface which is a process important for many hummocky moraines. Ablation till is less well consolidated and easily flows downslope when it first forms.