London Clay is an ideal medium for driving tunnels, which is why the London Underground railway network expanded quickly north of the River Thames, but south of the Thames the stratum at tube level is water-bearing sand and gravel (not good for tunnelling) with London Clay below, which is why there are few tube tunnels there. London Clay has a stand-up time long enough to enable support to be installed without urgency. It is also almost waterproof, resulting in virtually no seepage of groundwater into the tunnel.