Like most ancient buildings that have been added to from time to time, the abbey shows different styles of architecture, and the choir, which is early Norman, is undoubtedly the oldest part. The church consists of a choir with side aisles extending eastward for two bays, beyond which was an aisleless presbytery, the east end of which is demolished; a nave of nine bays, which had vaulted side aisles; a central crossing with square tower above; a north transept well preserved, and a south transept, of which the south end is destroyed.[324]

It has been suggested that the choir may have terminated with an eastern apse, but of this there is no proof. What survives consists of two bays next the crossing, the lower portions of which are in the Norman style. A unique arrangement is visible here, as far as Scotland is concerned, and resembles a somewhat similar design at Gloucester Cathedral and Romsey Church, Hampshire. The main piers have the peculiarity of being carried up as massive cylindrical columns to the arch over the triforium. The lower story has the round arch and vaulting ribs supported on corbels, projected from the round face of the piers. The triforium arch is round and moulded, and has a well-wrought chevron ornament. "It rests on large caps of the divided cushion pattern. The main arch is formed into two openings by a central round shaft and two half round responds, with massive cushion caps carrying plain arches."[325]

The clerestory is of Transition work, having one lofty stilted and pointed arch, and two smaller pointed arches in each bay. When the Transitional clerestory was erected, the eastern part of the choir is thought to have been built, and the remains of two lofty pointed windows are preserved to the east of the cylindrical piers. The same Norman style of architecture as in the choir is continued in the south and north transepts, and appears to have originally also extended into the nave. "This is apparent from the mode in which the string-course over the triforium runs along on the north side from the choir to the nave, where it is broken off. That the Norman nave has probably extended westwards from the crossing is further evidenced by the existence of the west end wall, with its great doorway and windows, and the south doorway to the cloister, which portions are all of characteristic Norman design." The Norman work must have preceded the Transition work in choir and nave by a considerable portion of time. There is no gradual development visible.

The nave (129 feet in length and 27½ feet in breadth) "is divided into nine bays, each of which comprises a main arch resting on clustered piers, a triforium with one round arch containing two pointed arches, and a clerestory forming a continuous arcade, with four pointed arches in each bay. The main clustered piers contain four principal shafts at the angles, and four intermediate shafts between them. The former are brought to a point on the face, the latter are flatter. The caps are simple and of an ordinary transitional form, each with a square abacus. The bases are also simple, and stand on a massive square plinth, a feature not uncommon in Norman work. The arches of the main arcade are somewhat acutely pointed, and the mouldings are bold, and resemble first pointed work."

The clerestory shafts are of trefoil section; the arches are all pointed, and contain first pointed mouldings. The west end of the nave and doorway are Norman in character, and Sir Gilbert Scott declared the great western doorway and south doorway to be "perfect gems of refined Norman of the highest class and most artistic finish." The doorpiece is surrounded by three gablets, the central one still retaining a trefoiled arch. The west wall has flat buttresses of Norman character, and "the upper portion of the wall has a central round-headed window, flanked on each side by three small pointed arch heads, the caps carrying which rested on long single free shafts, now gone. The central window has deep mouldings, but no enrichments. The west front has been finished with an octagonal turret on each side, as at Kelso Abbey, and the gable contains a central circular window, which has been filled with tracery at a late date. The west end walls of the aisles have each contained a circular-headed window of Norman design, with a chevron ornament in the arch and a nook shaft at each side."

"The lower part of the walls of the choir and the western wall and doorway and south doorway being all of Norman work, it seems probable that the whole building was set out and partially executed in Norman times, and that the work was either stopped for a considerable period and then resumed, or that the structure, after being completed, was destroyed, and had to be restored in the late Transition style. The Transition work is well advanced in style, and may be regarded as being of the date of the end of the twelfth century or beginning of the thirteenth century."

"The Norman north transept is fairly well preserved, but both the north and south transepts have undergone great repairs about the end of the fifteenth century. The crossing appears to have been so greatly damaged by the assaults of the fifteenth century that it was found necessary to rebuild it. The restoration is distinctly visible in the south-east pier of the crossing, the style of which is quite different from that of the Norman work adjoining in the choir and south transept, and the junction of the new work with the old is very apparent. This pier has clearly been rebuilt. It is plain next the crossing, but next the aisle it consists of a series of shafts with a moulded cap of late date. The upper mouldings of the cap form a continuous straight line, while the bells of the caps are broken round the shafts—a style of cap common in Scotland at the end of the fifteenth century."

"This pier and the south aisle of the choir beside it appear to have been restored by Abbot John Hall (appointed 1478), whose name occurs on the pier and on one of the bosses. The south-west pier of the crossing has also been rebuilt. This work was carried out by Abbot Thomas Cranston (appointed 1482). On a shield on this pier are carved the arms and initials of Abbot Cranston—three cranes and two pastoral staves—saltierwise. The same abbot's initials are placed on the north side of the west arch of the crossing, where the chamfer begins, and on the lower part of the north-west pier. The south-west pier, the north-west pier, and the arch between them would thus appear to have been rebuilt by Abbot Cranston. The base inserted by him is different from the old Norman base.

"About half-way up the south-east pier, rebuilt by Abbot Hall, the springer of an arch may be seen projecting to the west. Abbot Hall had evidently intended to throw an arch across the transept at this point, but Abbot Cranston changed his plan and the arch was not carried out. The mouldings of the portions executed by the two abbots differ in their respective parts of the structure.

"To the north of the original Norman north transept an addition to the transept has been erected. It is cut off from the old transept by a wall, and thus forms a separate chapel, measuring 27 feet in length by 22 feet in width internally. This chapel is vaulted with the pointed barrel vault usual in Scotland in the fifteenth century, and, consequently, the side windows are low, their pointed arch being kept below the springing of the vault. The window in the north end wall, however, is of large dimensions. The windows are all filled with good fifteenth century tracery, similar to that in the restored south aisle of the choir. This part of the edifice is now used as a mortuary chapel for the family of the Marquess of Lothian. The tower over the crossing is 33 feet square and 86 feet in height. It contains three pointed and cusped lancets on each side, and is without buttresses. It appears to have been erected about 1500. At the top, near the north-west corner, are engraved the arms and initials of Abbot Robert Blackadder, who was afterwards promoted to the offices of Bishop and Archbishop of Glasgow. He was appointed to that see in 1484, and died 1508. His arms are a chevron between three roses."