Umbrella trees

A string of umbrella thorn trees in Johannesburg.   Full name Vachellia tortilis, previously known as Acacia tortilis.  These ones are probably a little bit out of their more comfortable rural habitat.  This link explains why they're flat topped.

The genus Vachellia was named after George Harvey Vachell (1789 – 1839), chaplain to the British East India Company in Macoa (1825 – 1836), who reportedly collected plants in China.  It made him famous.