Description of logs from elderly lady crossing a jetty? (8)

'description of logs' is the definition.
(John Napier originated logarithms)

'elderly lady crossing a jetty?' is the wordplay.
'elderly lady' becomes 'nan' (informal term for grandmother).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'jetty?' becomes 'pier' (I've seen this before).
'a'+'pier'='apier'
'nan' enclosing 'apier' is 'NAPIERAN'.

'from' acts as a link.