Wikibooks: Historical Geology/Glacial marine sediment

• Format: Book
• Language: English
• Subjects: Antarctic; Arctic; Antarc*
• Online Access: https://en.wikibooks.org/wiki/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 un[[rounded]] or poorly ...