The landscape is dominated by the chalk hills of the Wiltshire Downs to the south and east. The Old Town stands on a hill of Purbeck and Portland stone; this was quarried from Roman times until the 1950s. The area that was known as New Swindon is made up of mostly Kimmeridge clay with outcrops of Corrallian clay in the areas of Penhill and Pinehurst. Oxford clay makes up the rest of the borough. The River Ray rises at Wroughton and forms much of the borough's western boundary, joining the Thames which defines the northern boundary, and the source of which is located in nearby Kemble, Gloucestershire. The River Cole and its tributaries flow northeastward from the town and form the northeastern boundary.