Historical Geology/Glacial marine sediment
In this article we shall discuss the deposition of glacial till in a marine environment. Readers may find it useful to refresh their memory of the relevant terminology by looking back at the article on glaciers.
Deposition of glacial marine sediment
The mechanism by which glaciers form and move has already been discussed in the main article on glaciers.
In that article we discussed the sediment deposited by the ablation of glaciers on land. Glacial marine sediment is formed when instead glaciers ablate into the sea: the rafts of ice so formed are carried out to the open sea by currents, carrying glacial till with them; when they eventually melt, they deposit this sediment.
It follows from their mode of transportation that although glacial marine sediments are composed of the same sort of material as glacial sediment on land, this material will be arranged quite differently: on land, we find the coarser material arranged in moraines, while the finer material is carried off by meltwater and spread across outwash plains. At sea, by contrast, the coarser sediment will be deposited at random among the finer sediment.
Glacial marine sedimentary rocks: how do we know?
We can observe present-day glacial marine sediment in the Arctic and Antarctic. The areas marked in cyan on the map to the right show where glacial marine sediments are presently being deposited. These sediments have sufficiently distinctive characteristics that when we find rocks with the same characteristics in the geological record, we are entitled to deduce that they are lithified glacial marine sediments.
The marine fossils in such rocks identify them as being marine. What identifies them as glacial is their unique structure. This consists principally of finely ground rock flour, often lacking layering, scattered throughout with dropstones: boulders and cobbles which have been rafted out to sea on glacial ice and then sank when the ice melted.
Being glacial material, dropstones are often unrounded or poorly rounded. Furthermore, some of them will betray clear signs of their glacial nature by being polished and striated as a result of being dragged along at the base of the glacier.
These features allow us to identify glacial marine sedimentary rocks. Our ability to do so helps us to find out about climatic conditions in the past; we shall discuss this further when we address paleoclimatology.