the Dumbuck shoal near Dumbarton. After James Watt's 1769 report describing this problem, a jetty was constructed at Longhaugh Point to block off the southern channel. This turned out to be insufficient to solve the problem, so in 1773, a training wall called the Lang Dyke was built on the Dumbuck shoal to stop water flowing over into the southern channel of the river. In the late 18th and early 19th centuries, hundreds of jetties were built out from the banks of the river between Dumbuck and the Broomielaw quay in Glasgow proper. In some cases, this construction had the effect of deepening the river, because the increased flow of the newly constrained water wore away the river bottom. In other cases, dredging was required to deepen the river. In the mid-19th century, engineers took on the task of dredging the Clyde much more extensively. They removed millions of cubic feet of silt to deepen and widen the channel. The major stumbling block encountered by that project was a massive geological intrusion known as Elderslie Rock. Because that rock increased the project's difficulty, the work was not completed until the 1880s. Around this time, the Clyde became an important source of inspiration for artists, such as John Atkinson Grimshaw and James Kay, who were interested in painting scenes that depicted the new industrial era and the modern world. Shipbuilding and marine engineering The completion of the dredging was well-timed, because the channel finally became navigable all the way from Greenock to Glasgow just when the steelwork industry had begun to grow in the city. Shipbuilding replaced trade as the major activity on the river, and shipbuilding companies started rapidly establishing themselves there. The Clyde soon gained a reputation for being the best location for shipbuilding in the British Empire, and grew to become the world's pre-eminent shipbuilding centre. The term Clydebuilt became an industry symbol of high quality, and the river's shipyards were given contracts to build prestigious ocean-going liners, as well as warships. The Queen Mary and, in later years, the Queen Elizabeth 2 were built in the town of Clydebank. Between 1712, when the Scott family's shipyard was built at Greenock, and the present day, over 25,000 ships have been built on the River Clyde, its firth, and its tributaries, the River Kelvin and the River Cart, by many boatyards, including those at Maryhill and Kirkintilloch on the