Welcome Guest [Log In] [Register]

From Aug 2006 - Nov 2013 WeDig provided a live forum for diggers & fans of Vindolanda. It has now been mothballed and will be maintained as a live archive.

Here you will find preserved 7 years of conversation, photos, & knowledge about a site many people love. Vindolanda gets under the skin. (Figuratively and literally as a volunteer excavator!) It's a place you remember, filled with people you remember!

Thanks for 7 great years!

ZetaBoards - Free Forum Hosting
Enjoy forums? Start your own community for free.
Learn More · Sign-up Now
Viewing Single Post From: Mike's Geoblog
Mike McGuire
[ *  * ]
The rocks and landscape around Vindolanda are the product of two ice ages, one in which the rocks were laid down and one in which they are being worn away.

An ice age is an extended period of time (millions of years) during which there are large ice sheets on land. At present there are ice sheets on Antarctica and Greenland, so we are currently in an ice age which has already lasted for over two and a half million years. The previous ice age was much longer, from perhaps 330 to 260 million years ago during the later Carboniferous and early Permian periods.

It’s a characteristic of ice ages that the amount of ice on land, and hence the amount of water in the oceans, varies cyclically over periods of 10s to 100s of thousands of years. For the past 15,000 years we have been in an interglacial, with relatively small ice sheets and high sea levels. But for nearly 100,000 years before that ice covered large parts of the northern continents and sea levels were about 100 metres lower.

During the Carboniferous and Permian periods nearly all the land masses of the Earth were gathered together in a “supercontinent” geologists call Pangea. The south pole was well within Pangea, and in the ice age large ice sheets waxed and waned around it, but the north pole was within the surrounding ocean named Panthalassa. Britain was also within Pangea, close to the equator, but much of it was near the edge of a seaway connected to Panthalassa, with mountains to the north and northeast.

When sea level was high, a wealth of sea creatures lived in the warm, clear water. When they died their shells and skeletons collected on the sea floor and over time were converted into limestone. As sea levels started to fall, the water became muddy and most of the sea creatures died out. Great depths of mud accumulated which became compacted into shales. Eventually the water became very shallow and sand was deposited in a beach, delta or estuary environment. Finally, in some cycles, marshes developed with large plants and trees which fell into the water when they died and were slowly converted into coal. These cycles of limestone, shale, sandstone and coal occur over much of northern England and are called Yoredale Cycles after the old name for Wensleydale where they were first described. In the Vindolanda area they became tilted and dip at an angle of about 12 degrees to the south southeast.

Over the succeeding millions of years the Earth gradually resumed its more normal warmer state and many layers of other rocks accumulated over the Carboniferous ones. But once the ice sheets of our present ice age started to flow over the area the moving ice ground all the rocks away and the Yoredale cycles are once again exposed. During the later part of the last glaciation the ice movement was from west to east, along the grain of the dipping rocks. This left the harder rocks, the limestones and especially the sandstones, sticking up as north facing escarpments and the shales were worn down further to leave small valleys partly filled with boulder clay. The largest escarpment is that of the Whin Sill (see last week’s blog). There are places, particularly along the military road, where up to a dozen of these escarpments can be seen apparently marching across the landscape like waves approaching shore. As the last ices sheet retreated, perhaps 14,500 years ago, huge amounts of meltwater cut spillways through the escarpments as well as valleys like those which surround Vindolanda on three sides.

So the geology of two ice ages provided the Romans with:-
• the Whin Sill escarpment to build their wall on
• sandstone for building their walls, forts, vici, bathhouses, etc, etc
• limestone for mortar
• clay for wall bonding, tiles and pottery
• coal for fuel
• a defensible site at Vindolanda.

Remember, oh digger, when the icy east wind blows across the site, do not complain about the cold because without cold and ice in the past the Romans would not have built Vindolanda for you to excavate.
Offline Profile Quote Post
Mike's Geoblog · Excavation & General Archaeology Discussions - Open to All!