Track ballast is the crushed rock which supports and restrains railway track and provides effective drainage. Some of the qualities looked for in the rock selected for ballast are its resistance to abrasion which occurs when trains pass, and its ability to resist breakdown from environmental factors. Detailed investigation has been undertaken of a site known as Swansea Ridge for CP Rail System. The Swansea Ridge site offers superior rock quality, acceptable geology, minimal impact on the environment, space for development and its location close to the railway's track. Based on an estimated annual requirement of 362,840 tonnes of ballast, Swansea Ridge will supply ballast to CP Rail System in eastern British Columbia, southern Alberta and parts of Saskatchewan for about 50 years. The proposed quarry will consist of four main operations: mining rock, crushing rock to produce ballast, stockpiling ballast and loading ballast into rail cars. Diabase sill material will be the source of the track ballast.
Site geology has been compiled and interpreted from test pit data, diamond-drill hole data and surface exposures. Geology maps indicate the area is underlain by Helikian Purcell Supergroup rocks and Proterozoic Moyie intrusions. Four structural units were recognized. From top to bottom they are: overburden, cap rock consisting of bedded quartzite and argillite, diabase sill and a lower sequence of bedded quartzite and argillite.
The metasedimentary cap rock which overlies the sill and the metasediments underlying the sill belong to the Aldridge Formation of the Helikian Purcell Supergroup. These rocks are predominantly composed of bedded quartzite, biotitic quartzite and argillite. The rocks generally strike along the length of Swansea Ridge and dip roughly parallel to its northeast-facing slope. The thickness of cap rock across the length of the ridge is variable. It reaches its greatest thickness on lower slopes on the northern portion of the ridge, where in excess of 60 metres overlies the sill. On the southern portion of the ridge, it has been completely eroded. Cap rock has also been completely eroded along the crest of the ridge.
A Moyie diabase sill has intruded the Aldridge Formation and lies concordant with bedding. The rock is comprised of interlocking grains of unaltered plagioclase feldspar and hornblende. Two phases are recognized; a coarser grained phase at the top of the formation and a medium grained phase in the middle and lower sections. The thickness of the sill is consistent across the length of the ridge and averages approximately 60 metres (true thickness).