As these two Sections of the M6 were constructed within the same timescale, they are considered together. The initial survey, preparation and design was undertaken by the Cumberland County Council as the Minister's Agent Authority. The North Western Road Construction Unit (RCU) was formed on the 1 April 1967. Unlike the County Councils of Cheshire and Lancashire, Cumberland did not participate and completed the design and undertook the supervision of construction, with the County Surveyor acting as Engineer under the various Contracts.

From the interchange at the northern end of Penrith By-Pass at Catterlen, the selected route for the Penrith to Carlisle Section was along the valley of the River Petteril a tributary of the River Eden. To the west of the West Coast Main Line Railway it was also within the same corridor as the A6 lying to the east. Passing through an agricultural belt of pasture, arable, meadow and woodland, the 12 mile route provided excellent vistas of the Pennine range.

It was proposed that a Service Area would be provided near to the village of Southwaite.

Alternative routes for Carlisle By-Pass were examined both to the east and west of the City, having regard to the alignment of both the A69 to Newcastle, and the A595 linking West Cumberland. However, the existing urban development suggested an eastern route as being preferable, in order to avoid major disruption, although this meant the postponement of improvements to the western approaches to the City.

From a two-level interchange connecting with the A6 at the Golden Fleece, the proposed 7 mile route was to pass through a series of cuttings, and under the A69 where a further two-level interchange was to be provided at Rose Hill. Emerging into the Eden valley, with a down-grade to the River flood plain, distant views of the Scottish Lowlands could be seen. After crossing the River and the line of Hadrian's Wall, the motorway was to terminate north of the City at the Greymoorhill Interchange close to the junction of the Carlisle-Edinburgh Trunk Road A7 with the Carlisle-Glasgow Trunk Road A74.

From this point, it was proposed that the existing A74 should be reconstructed over a length of 4½ miles in order to provide a high standard dual carriagway all-purpose road extending as far as the crossing of the River Esk, approximately one mile south of the Scottish Border. The new 604 feet long Esk Bridge was to be the latest replacement for the iron structure originally built by Telford and for a more recently constructed reinforced concrete bridge which, each in turn, had proved to be inadequate for the increased traffic needs. Eight other bridges were required in the reconstruction.

There were similar consultation meetings with members of the public and major landowners to those undertaken for Penrith By-pass. Again, the statutory procedures for the motorway were completed without undue difficulty. The only Public Inquiry to be held, was in respect of the Southwaite Service Area, when fears were expressed at the possibility of intrusion by way of noise and lighting.

A number of advance works contracts were undertaken, the most significant being the Low Hurst Railway Bridge designed by British Rail to carry the West Coast Main Line over the motorway at the northern end of the Penrith to Carlisle Section. East of the City, two other bridges were constructed in advance, to carry the motorway over the Skipton-Carlisle and the Newcastle-Carlisle Railways. Other advance works involved drainage and earthworks, including a surcharged embankment built on the alluvium of the Eden floor plain.

With a population of 75000 in the City, priority was given to the constructions of the Carlisle By-pass section, in order to provide early traffic relief. Work on the main Contract began in December 1968.

Tenders had been invited for alternative forms of construction for the dual three-lane carriageways namely, 'flexible', 'composite' and 'rigid'. The lowest tender, which was accepted, allowed for composite construction but, in the event, the Contractor offered to provide a flexible pavement at no extra cost which resulted in the whole of the M6 through Cumberland having this type of carriageway.

The designed profile had aimed at a balance of cut and fill in carrying out the earthworks, having regard to the disposal of unsuitable material. In a similar manner to the practice adopted in the construction of Penrith By-pass, a 12 inches thick capping layer of bunter sandstone was to be provided below the sub-base.

Apart from those constructed in advance, a further 19 bridges were required, the most significant being the eight span 720 feet long structure carrying the motorway over the River Eden. Preflex Beams composite with a reinforced concrete deck permitted rapid construction.

Where suitable boulder clays and sandstones were encountered at economic depths, the structures were supported on strip footings, otherwise foundations were piled. Wide use was made of Universal beams composite with reinforced concrete decks of up to five spans, carried on piers, bank seats and, in some cases, sloped abutments.

The By-pass was opened to traffic in December 1970.

A separate contract was awarded for the A74 reconstruction and work had started in September 1969 with provision for completion of the Greymoorhill Interchange by the same date as the opening of the By-pass.

Meanwhile, in March 1969, work had begun on the main Contract for the 12 mile Penrith to Carlisle Section which was to be constructed to similar standards.

The boulder clay was found to have a high silt content, thereby making it more susceptible to the adverse effect of wet conditions. The higher pore water pressures made it necessary to make extensive use of bunter sandstone as imported fill, together with the provision of additional drainage. A volcanic intrusion known as the Armathwaite Dyke was encountered, as expected, but caused no particular difficulty as the rock was largely fragmented.

In addition to the advance railway bridge at Low Hurst, 19 other bridges were required. Apart from the use of precast concrete pre-tensioned beams, many of the bridges are to trapezoidal steel box construction. During the design and construction period, considerable development and changes in the design 'rules' took place, most notably leading to modifications to the steel box structures, following the Merrison Report. This resulted in additional diaphragms and stiffeners being inserted in to some of the bridges at a late stage of construction.