The "Dark Ages" saw a reversion to fords, and new bridges were often built on their sites rather than on the locations of earlier Roman bridges.
"A road map of the eighteenth or even the early twentieth century may provide a more accurate picture of the routes of late Anglo-Saxon England than the Roman roads which are usually depicted."There was major investment in transport in the two centuries after the Norman Conquest, with bridges among the principal foci of a new road system.
"Not only it is certain that a high proportion of bridges existed by 1350, but there is good reason to suppose that many of them were there by the early thirteenth century or even before."But how much of this involved the construction of new bridges is uncertain:
"it is highly likely that some of the many bridges standing in the twelfth and thirteenth centuries had been constructed by the late eleventh century, but we do not know how many bridges this was."
Where more information is available, Harrison provides some details and links bridges to broader history.
"By the late Saxon period we can trace what was probably then the main road from London to the north through Nottingham and Doncaster. Domesday Book refers to a section of it as the road towards York in Nottinghamshire. It passed through Blyth and Doncaster on its way to York. This section of the road may already have been in use in 868 when the Danes, based in York, seized Nottingham. Here there was a ford of the Trent. Just over fifty years later the importance of the route was firmly established by the construction of the bridge which channelled north-south traffic to this key crossing. It was by far the most important crossing of the Trent in the eleventh century. William the Conqueror used this road when passing to and from the north on his major campaigns in 1068 and in the spring and autumn of 1069."
The 500 years after 1250 were a period of relative stability.
"In the eighteenth century, and especially after 1760, the situation changed again. New bridges were erected at many sites. The remaining ferries on secondary roads were replaced by bridges. This was the second great age of bridge construction, which has lasted to the present day. Nevertheless, the main river crossings were still, with few exceptions, the same as they had been in the middle ages."
Part II turns to bridge structures. Harrison begins with the threats faced by bridges, most notably from extreme floods. The basic construction options were timber, stone and timber, or arched stone; evidence for these comes from illustrations, written descriptions, archaeology, and inspection of surviving bridges.
Harrison gives a history of bridge engineering: early timber deck bridges and causeways, the introduction of vaulted stone bridges, the use of coffer dams in construction, developments in foundations and arches, and so forth. He also looks at regional variations:
"during the period from the eleventh to the fifteenth centuries there were opposing tendencies in different parts of the country. Most lowland bridges in the south and Midlands remained of similar design and construction for centuries. Elsewhere medieval masons made significant advances: the principal were the ability, first, to devise foundations for even the most difficult conditions; and secondly, to erect very big arches, and to recognize the value of and to construct large segmental arches."
It has been argued that medieval bridges were of low quality and poorly maintained, but Harrison weighs the overall evidence differently:
"For the most part, by the end of the middle ages bridges were well built, substantial structures. Routine maintenance and repair was common. When bridges were out of repair they often remained useable, and when part of them collapsed they could be patched up. If the damage was very serious, temporary bridges could be erected until rebuilding had taken place."
Part III looks at the funding of bridge construction and upkeep. Records of costs are better for the later period, but even in earlier periods substantial resources were devoted to bridges. Funding came from bridgework obligations, charitable donations and tolls; by 1900 county councils had mostly taken over responsibility for bridges, but
"at the beginning of the motor-car age bridges were still being repaired by landowners who had inherited a liability almost a thousand years old, or from funds from bridge estates which had been established almost as long".
The Bridges of Medieval England includes sixteen pages of black and white photographs of bridges. It assumes a basic familiarity with English history and with architectural terms such as "ashlar", "voussoir", and "soffit". It also gives short quotations in Latin without translation. But mostly it is more accessible, offering an appealing integration of history, architecture, engineering, and human geography.