Tunnel are not impossible in South London, but they are expensive and they are proportionally more expensive over short distances. As somebody I spoke to at TfL once put it; once you start tunnelling you might as well keep going.

Returning to geology, this map shows that there is no significant difference between north and south London or the Kent coast at Dover.



Improvements in tunnelling methodology even in the last 20 years mean that tunnels can be built deeper and quicker. Tunnellers learned a lot from the Channel Tunnel and the Jubilee Extension about rapid building of tunnels in South East England's geology. Alternatively you could use cut-and-cover techniques but this would mean closing Stanstead Road for a few years while the DLR is enclosed beneath.

But if you are tunnelling to Forest Hill it is better to come down the Bakerloo line rather than coming from Catford. If there is a need for more transport systems going directly to Canary Wharf then I would extend ELL phase 2 from Surrey Quays directly to Canary Wharf rather than relying on the Jubilee Line interchange at Canada Water. This does mean boring a short tunnel but the value in terms of passengers carried would be significant.