University:
University of Edinburgh
Course:
EASC08030 | Geology and Landscapes
Academic year:

2020
Views:

289

Pages:

36
Author:

holly2185g8xw

Welcome! Beach Pebbles: Windows into Scotland's Geological Past Welcome! Beach Pebbles: Windows into Scotland's Geological Past Scotland has incredible geological variety:  wide range of rock types  formed over a large part of Earth history (oldest fragments are 3300 million years old, youngest rocks just 55 million years old)  this geology creates Scotland’s landscape and intricate coastline  and the coast is the best place to see Scotland’s geology! Simplified geological map of Scotland (with key) available from scottishgeology.com What is a pebble? Wentworth (1922) Grain Size Classification Boulder >256 mm Cobble 64–256 mm Pebble 4-64 mm Granule 2-4 mm Sand <2 mm Where do pebbles come from? Pebbles form by weathering of bedrock – physical breakdown of exposed rock surfaces. Many pebbles are local, derived from rock exposures where weathering processes (e.g. freeze-thaw, wave action or plant roots) have created rock fragments. But we also have to consider the processes that can move pebbles: waves, rivers, ice. Pebbles can be any of the three types of rock, but they have different characteristics: Sedimentary Original rock often layered, which can control pebble shape Igneous Tough, crystalline rocks, usually without internal structure Metamorphic Rock altered by heat and pressure, might inherit features of original rock or be dominated by new alignments Sedimentary rocks Sedimentary rock forms when sediment (e.g. sand and mud) accumulates at the surface and gradually gets buried. The sediment has usually been transported by water or wind, and deposited in flat layers (beds). Most sediment grains derive from erosion of older rocks; but sedimentary rocks also form from animal and plant fragments or chemical precipitation. Pebbles of sedimentary rock are often pretty soft and easy to erode. Because of the internal layering, the pebbles are often flattened. Sedimentary Pebbles Rackwick Bay, Hoy, Orkney Igneous rocks Igneous rock forms when magma (molten rock) cools and crystallises to solid rock. The magma comes from melting of existing rock. Igneous rocks include: (1) volcanic rock that forms at the surface when magma is emitted as lava or ash from volcanoes. e.g. basalt, tuff. (2) plutonic rock formed underground, where magma cools more slowly, forming tough, crystalline, coarse-grained rocks. e.g., gabbro, granite. Igneous rocks don’t have any internal alingment to control pebble shape. They are usually fairly tough, and so may have an angular or irregular shape. Igneous Pebbles Eroding basalt columns, Staffa Metamorphic rocks Formed by distortion of existing rocks, by burial and/or heating. Metamorphic effects range from slight changes to the rock fabric, to complete modification of the mineral content and structure of the rock. e.g., schist, gneiss, slate, marble. Metamorphic pebbles may inherit layering from the original rock, producing flattened pebbles. Or they might be shaped by an imposed fabric due to regular orientation of crystals within the rock. Metamorphic Pebbles Moine metamorphic rock pebbles, Loch nan Uamh, Lochaber Local pebbles Cairngorm mix – granite and schist Survival of the toughest The pebbles on a beach are not necessarily representative of the variety of local rock types. Particularly when sedimentary rocks are mixed with harder rocks, the toughest survive and will be represented in greater proportions on the beach. Singing Sands (Camas Sgiotaig) Isle of Eigg Rackwick Bay, Hoy, Orkney And not all pebbles are local! Loch Buie, Isle of Mull Billion-year-old pebbles Torridonian sandstone, Stac Pollaidh, Assynt Pebbles as windows to the past Beach Pebbles Moine metamorphic rock, Knoydart Triassic sandstone, Elgin Beach Pebbles Scotland’s pebble beaches – some examples Discover the variety of pebbles found on different beaches around the country. Think about what influences this variety – both between beaches, and on an individual beach. Rainfall Amount A1nnu:al Average 198,1-2010 Average Valu:e (mm) .. >3000 2000tоЗООО • 1500 to 2000 -· 1250 to 1500 1000 to 1250 800 to ООО 700 to 800 "600to700 .. <600 Beach Pebbles Successive ice sheets during the Quaternary (last 2.6 million years) have been responsible for moving rock across Scotland. Maximum ice thickness on the watershed means shorter transport distances to the west. The Blue Stone, Fife Ness Image from BritIce project Moine metamorphic rock pebbles, Loch nan Uamh, Lochaber Columba’s Bay, Iona – local metamorphic plus erratics Funzie conglomerate, Fetlar, Shetland Near the Highland Boundary Fault – Dunotter Castle, near Stonehaven Beach Pebbles Siccar Point, Scottish Borders – junction between two sets of sedimentary rocks Junction of Greywacke and Old Red Sandstone, Siccar Point Beach Pebbles Mix of Greywacke and Old Red Sandstone, near Siccar Point Isle of Rum – local geological variety reflected in varied beaches Guirdal Bay, Isle of Rum Harris, Isle of Rum Beach Pebbles Raised storm beach Pebble Pebble, by Zbigniew Herbert 1924-1998 a pebble is a perfect creature equal to itself mindful of its limits filled exactly with a pebbly meaning with a scent that does not remind one of anything does not frighten anything away does not arouse desire its ardor and coldness are just and full of dignity I feel a heavy remorse when I hold it in my hand and its noble body is permeated by false warmth —Pebbles cannot be tamed to the end they will look at us with an eye calm and very clear Translated into English by Peter Dale Scott