This earthcache takes you on a tour of some of the newer buildings in Cambridge city centre and covers about 0.6 miles. It can be completed in around an hour on foot, not including the time required to properly admire the history and architecture of this incredible place. Much of the route is not accessible to vehicular traffic but is suitable for bicycles, pushchairs and wheelchairs. Paid parking is available nearby. No point on the tour is more than 10 mins walk from market square. Refreshments are available along most of the route. The walk takes you past or near several other caches on your way around.
You will need an accurate length measuring device in order to complete the tasks. This cache can be combined with Cambridge Stones – Water to form a loop. You can do them in either order but this one is chronologically second.
The Cache
The stones that you will see along the way were all formed in some way that involved magma, except for those in the Grand Arcade. They represent geological activity over a period of more than 250 million years. That staggering span of time makes the entire history of human existence seem insignificant in comparison. Most of the stones seen during the walk have been formed at the Earth's core and may have taken up to a million years just to cool before adorning these buildings. Don't forget to be awed when you touch them. Let's begin.
Note: Part of this walk takes you through the Grand Arcade, which has limited opening hours. You can check those hours here.
Proceed SE along Bridge St. For 180m. As you pass Sidney Sussex College on your left the street name changes to Sidney St. Continue to the intersection with Green St. On the S corner is The Edinburgh Woollen Mill building. The dark stone facing this building is larvikite, an igneous rock formed in the Permian Period, 251 to 299 million years ago, in Larvik, Norway. The two minerals visible in this stone are small dark crystals of augite and large, irridescent potassium feldspars. These crystals are formed when magma rises up through the Earth's crust and remains near the surface, cooling slowly over up to a million years.
Task 1: Measure the lengths of the potassium feldspars and estimate their average length.
Continue along Sidney St. for 150m to the Marks & Spencer building, on the left. There are three different igneous rocks in the facade of this building: a white granite at the top above the entrance (not on the ground), a red granite at eye-level and black gabbro, a coarse-grained igneous rock typically composed of pyroxene and plagioclase feldspar, at pavement level. The origins of these stones are undocumented but the white granite is of Permian age, probably from Dartmoor, the gabbro is most likely from South Africa and the red granite is probably from Scotland or South Dakota, USA.
The red colouration of the red granite is partly due to the presence of pink feldspars but largely due to the iron oxide staining around the edges of the crystals.
Task 2: Measure the lengths of the pink feldspars in the red granite and estimate their average length. Compare the sizes of the feldspars in the white, red and black granites.
Continue along Sidney St. to its intersection with St. Andrew's St. and turn right. On your left is Christ's College. Pause and admire the wonderful old college gateway. Continue SE along St. Andrew's St. for 300m. Just past the bus stops on the right, enter the Grand Arcade.
The arcade is clad inside and out with Jaumont Limestone from Lorraine, France. You would be forgiven for thinking that the floor here is filthy but look again. It's chock full of fossils. The main flooring stone is a limestone quarried from Treuchtlingen, Germany, the same quarry where the famous fossil of the feathered dinosaur Archaeopteryx was found! It was formed during the Jurassic Period, from 145.5 to 199.6 million years ago, and contains numerous fossils of squid-like belemnites, spiral-shelled ammonites, brachiopods (shellfish) and various sponges.
Proceed straight ahead (SW) on the ground floor and turn right (NW) at the end. If you stand with the escalator to your right, you will have the blank wall between the first two shops on your left next to you. Continue for 20m or so to the area of wall between the third and fourth shops on the left. In the floor at the base of the wall between the shops is a large ammonite fossil. Just ahead on the right is a very suitably named shop.
Task 3: Measure the diameter of the ammonite.
Continue NW along the ground floor and exit 30m on the left into Fisher Square. If you miss the first exit there is a second that also leads into Fisher Square. The large boulder featured here is a red granite glacial erratic from Finland which has been used as the basis of a sculpture named "Between the Lines". Glacial erratics are boulders that were picked up and moved, possibly many miles, by a glacier.
Proceed out of Fisher Square into Corn Exchange St. Turn left and proceed to the end, where it intersects with Downing St. On the opposite side of the street and to the right is the entrance to the Sedgwick Museum.
The entranceway is paved with pink granite. Some of the paving slabs have darker patches of rock called xenoliths. These are older rocks that were incorporated into the cooling granite while it was still liquid. Some are broken off by the passage of the magma as it rises within the Earth's crust and others may have fallen into the magma from the roof of the magma chamber. There are many xenoliths visible in the paving and kerbs around Cambridge.
Task 4: In the central archway, in line with the front wall and under the closed position of the left-hand gate you will see a large xenolith in the granite. It is roughly rectangular with one end rounded and the other truncated by a join in the pavement. Measure the length of this xenolilth.
This walk ends here. From here you can continue along Downing St. to Trumpington Rd or St. Andrews Rd, retrace your steps to a suitable point or continue your journey with Cambridge Stones – Water.
To log this cache, please log your find normally and then email me your answers by clicking on the envelope icon at the top of the page next to my user name. Please enable the option to include your email address. I will contact you if your answers need clarification. Feel free to include photos of your journey as there is no shortage of opportunity for great photos in Cambridge but, as usual, please DO NOT include the answers or related information in your log or photos.
Terminology
Igneous rocks are the solid remains of magma which has formed deep inside the Earth and risen towards the surface through the Earth's crust.
References
The Sedwick Museum of Earth Sciences. Cambridge Geology Trail. Retrieved 6 August, 2012.
Nigel Woodcock; David Norman (20 Aug 2010). Building Stones of Cambridge: A walking tour around the historic city centre. Retrieved 6 August, 2012.