did not cave in because the ground was frozen. Miners discovered old underground beach and river gravels rich with gold. Around 1900 the population of Nome was more than twenty thousand, many of them drift miners. Nome's gold fields, appearing untouched from the surface, are honeycombed with tunnels left by the gold rush drift miners. Today's miners, prospecting with modern drilling equipment, sometimes hit old drifts; this was, and is, a technique copied from the Welsh coal miners of south Wales and is much more effective than using bell pits. California Gold has been mined from placer gold deposits up and down the state and in varied environments. Initially, easily discovered deposits in surface and river placers were mined until about 1864. Hydraulic mines, using powerful water cannons to wash whole hillsides, were the chief sources of gold for the next 20 years. In 1884, Judge Lorenzo Sawyer issued a decree prohibiting the dumping of hydraulic mining debris into the Sacramento River, effectively eliminating large-scale hydraulic operations. For the next 14 years, drift mining placer gold deposits in buried Tertiary channels partially made up for the loss of placer gold production, but overall production declined. Production rose again with the advent of large-scale dredging. The first successful gold dredge was introduced on the lower Feather River near Oroville in 1898. Historical drift mines in other countries Drift mines in current production Safety and environmental issues Drift mines in eastern Kentucky are subject to roof collapse due to hillseams, especially within 100 feet of the portal. In 1989 the U.S. Bureau of Mines published a study of eastern Kentucky drift mines as part of an ongoing research program to characterize the outcrop barrier zone. "Hillseams" were identified as the dominant geologic cause of roof instability unique to the outcrop barrier zone, with many roof fall injuries and fatalities attributed to them. Hillseam is the eastern Kentucky miners term for weather-enlarged tension joints that occur in shallow mine overburden where surface slopes are steep. Hillseams are most conspicuous within 200 ft laterally of a coalbed outcrop and under 300 ft or less of overburden. Hillseams form by stress relief, and therefore tend to parallel topographic contours and ridges. They can intersect at various angles, especially under the nose of a ridge, and create massive blocks or wedges of roof prone to failure. Examples of hillseams are described in both outcrop and in