The Pembroke Breccia disconformably overlies the basal limestone of the Mississippian Windsor group in central Nova Scotia and has previously been considered a primary breccia in the Windsor group. The breccia is estimated to be as much as 100 feet thick. It is absent in deep borings; at depth, the basal limestone is apparently conformably overlain by interbedded limestone and anhydrite.

The Pembroke typically is a jumbled mass of unsorted angular limestone fragments in a matrix of muddy massive limestone. Most of the fragments are similar to the basal laminated limestone of the Windsor group, although some are of massive limestone and red marl apparently derived from beds higher in the section. Irregular pipes, channels, and masses of sandy breccia containing scattered quartzose pebbles occur within the typical breccia. In a few exposures the breccia contains relict beds; in others, thin graded calcarenite beds appear to fill pockets in the breccia.

Late Paleozoic regional deformation extensively folded the Windsor group; the Pembroke Breccia is younger than this deformation. Stratification in floored cavities is virtually horizontal and is independent of the attitude of the adjacent Windsor strata. The deformation produced veins normal to bedding in the Windsor limestone; many rotated fragments within the breccia contain similar veins normal to their bedding. Metamorphic fabric, absent in the breccia matrix, varies in nature and orientation from fragment to fragment. Sand grains, pebbles, and heavy minerals within sandy parts of the breccia appear to have been derived from Triassic rocks, suggesting that the breccia originated in either Triassic or post-Triassic time.

It is here proposed that the breccia formed only near an erosion surface by collapse and disintegration following solution of anhydrite interbeds and part of the limestone. The sandy breccia appears to be composed of surficial debris which filled solution channels within the typical breccia.

Because the Pembroke Breccia was formed long after the deposition of the Windsor group, it should not be regarded as a stratigraphic unit of the group. The actual depositional sequence of the basal Windsor is (in ascending order): limestone, interbedded limestone and anhydrite, anhydrite, and halite.

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