17.4 Glacial Deposits
During the most recent ice age—that is, the period of multiple glacial cycles that occurred between about 2.6 million and 12 thousand years ago—much of the North American and Eurasian continents were covered by glaciers. Expansion of the glaciers ripped up a considerable amount of rock and sediment, which was then carried over the continents. After the ice retreated, the glaciers left behind extensive deposits of glacial sediments. These deposits have a large range of grain sizes, from mud and silt to boulders the size of a building, which result in deposits that have a broad range of permeability. Modern glacial sediments can also be found worldwide in high mountainous areas. We group all glacial deposits—called drift—into two classes—unstratified and stratified, or layered.
Unstratified drift is called diamicton or till. Diamicton is a general term describing any large deposit of geological material that contains clasts of many different sizes that are not well sorted. This includes glacial deposits as well as mass wasting deposits—for example, mudflows, avalanches. Till is a specific diamicton of known glacial origin. It consists of mixed sediments containing sediment grains ranging from boulders to very fine clays. Till may be hard and compacted by the weight of glaciers that flowed over it. It is generally poorly sorted, although tills deposited upon or beside glaciers may be graded in glacial outwash channels.
Most well yields in till are very small. The poor sorting leads to low porosity and permeability. Productive wells must tap sandy layers or fractures in the till or else be in a very sandy till.
Stratified drift is comprised of glacial sediments that are deposited in layers. In contrast to till, these units can be very productive. Stratified glacial drift is generally formed by one of the processes as listed here.
- Outwash deposits have characteristics like alluvial systems because they are both formed by running water. Outwash can be found in vast aprons, or plains that spread out in front of a glacier. These are formed when melt water from the glacier runs out from beneath the ice and carries sediments from the glacier. Localized outwash deposits are found in pitted plains and as ice‑contact stratified drift. These tend to be smaller deposits than outwash plains and may be more poorly sorted. Outwash can also be intercalated—sandwiched or interbedded—between low permeability tills.
- Productive river valley (floodplain) type deposits are common in deglaciated areas. Some may be buried by glacial drift (0 to 150 m thick), while some form and are exposed at the surface in valleys downstream from glacier termini.
- Eolian deposits are sediments that were transported by wind. We identify two basic types of eolian deposits: dunes and loess. Sand dunes, if saturated, are excellent aquifers—they usually have well‑sorted fine sand sized sediment grains with little silt and clay. Loess is silt‑sized grains that are deposited in extensive layers and are generally not good aquifers, as they tend to have low porosities and permeabilities.
- Lacustrine deposits are also common in deglaciated areas. These form from small lakes that dry out, leaving behind deposits of very fine‑grained material. The deposits are thinly layered, very low‑permeability silts and clays that do not form productive aquifers. These can, however, form non‑continuous low permeability units that can support perched aquifers in overlying sandy units.