The answer lies in the fact that about 250,000 years ago the whole of Cambridge city as we know it now was covered by the floor of a river valley. It was very different to the river Cam that we see today; it consisted of lots of channels separated by small gravelly islands and banks that shifted all the time.

A braided river in New Zealand

Certain conditions can make this type of braided river 'jump' to a different course - particularly after violent melting snow floods, which is what would have happened to the river Cam in Ice Age periods.

Originally, the braided river flowed north along Hills Road, Regent Street, straight through the city centre, and past Castle Hill along Huntingdon Road - pretty much perpendicular to the present river! The gravelly river deposits found on top of Castle Hill actually used to form the bottom of the braided river valley... What was the bottom of a valley is now the top of a hill!

This startling landscape change is called 'inversion of relief' or 'inversion of topography'. It appears that the river gravels formed an armoured layer that slowed down erosion compared to the rapid erosion of the relatively soft bedrock (chalk and clay) of the surrounding valley sides.

Further along Huntingdon Road near the Beefeater pub and restaurant, is the Traveller's Rest Pit. This site produced worked flint tools, along with gravels from an earlier course of the proto-Cam which flowed here some 300 to 400,000 years ago. You can see artefacts from the pit in the Sedgwick Museum of Earth Sciences and the Museum of Archaeology and Anthropology.

This high ridge of gravel, stretching from Huntingdon Road, past the Beefeater, through Girton College and Girton village is one of the most ancient alignments of the ancient river Cam and can very reasonably be regarded as the 'great-grandfather' of the present Cam!

Looking up Castle Hill

A city is born
As the Romans first arrived in Cambridge in AD 70, they were travelling north from Colchester, trying to avoid the wet and marshy Fens to the east. They arrived in Wandlebury and looked down on a very, very muddy Cam Valley.

They would have seen Castle Hill in the distance, and rather than go round the muddy swamp below, they preferred to go in a straight line right across. Imagine them going past Addenbrooke's Hospital, along Regent Street, crossing the river at the Quayside, then up past Shire Hall and off towards Godmanchester!

So Cambridge began on top of Castle Hill as a Roman garrison guarding a river crossing on the road to the north. Having such a high view point was a strategic advantage for early settlers which is why the Norman's made the most of this natural hill by building a motte and bailey castle upon it. In fact you can still see the motte - the castle mound - now.

People began to live on the hill including the Saxons who later settled down on the river valley on the gravel and mud. Cambridge grew as a Scandinavian trading centre and flourished as an inland port. If you look at the city coat of arms you'll see ships, sea horses and bridges which are a reminder of the importance of the river. The town was founded as a river crossing and it flourished because the river was the equivalent of the M11 of its day!

A 360° view of the city!
Castle Mound is a rare example of chalk grassland close to a city centre. It's a great place to look out and see how green Cambridge is. It's also a great place to look out for birds including birds of prey, swifts, sparrow hawks and house martins. Grasshoppers and butterflies thrive on the sunny slopes and there's also an enormous population of small black and white snails. The trees that you can see from the mound include fruit trees, birch trees and sycamores - and the interesting-looking tree at the bottom is called a Cedar of Lebanon.

Walk down Castle Hill until you reach the cross-roads at the junction with Chesterton Lane. On the opposite side of the road you'll see a big tall monument.

The Saxon monument

About four years ago this part of the road had to be dug up. This site where the monument stands would have been just off the path of the Roman road, so rather than digging through 800 years of road, they were going through 1500 years worth of human habitation! As they went down they went through medieval houses and the archaeologists found gold treasure... perhaps some medieval merchant had hidden his stash and forgotten about it! At the bottom they found bones of a Saxon graveyard and it's a reminder that for centuries, Castle Hill was Cambridge. It was where people settled above the valley. Castle Hill began as the administrative centre for this area and it still is - with Shire Hall there today.

Turn left at the junction and keep walking along Chesterton Lane until you reach the pedestrian crossing where you can cross over the bridge to get to Jesus Green.