Auckland's area of highest population density and growth, also has a blacklog of infrastructure projects; thus requirements for quality rock aggregate to satisfy demands for housing, construction, and road building are increasing substantially. All prime local aggregate resources (young basalts) have either been exhasted or sterilised by urban expansion. The other locally available aggregate resource is greywacke, a collective term that includes argillite which the industry perceives to have a particularly high swelling clay content. Since swelling clays are a major source of poor aggregate performance, contract specifications for supply of roadbase aggregate for Auckland roads are often overwritten with the statement that it shall include no argillite, thus effectively ruling out greywacke quarries and forcing consumers to purchase from more distant source. The cost of road transportation from source to market now averages close to 50% of the average delivered cost. The high plasticty swelling clays impart to roadbase is minimised by modification or chemical stabilisation with lime, steelwork slag and other substances. Current research is focused on determining the nature of the swelling clays in aggregate source rocks and the affect they have on the materials properties of aggregates