  Highly frustrated systems have macroscopically degenerate ground states that
lead to novel properties. In magnetism its consequences underpin exotic and
technologically important effects, such as, high temperature superconductivity,
colossal magnetoresistence, and the anomalous Hall effect. One of the enduring
mysteries of frustrated magnetism is why certain experimental systems have a
spin glass transition and its exact nature, given that it is not determined by
the strength of the dominant magnetic interactions. There have been some
suggestions that real systems possess disorder of the magnetic sites or bonds
that are responsible. We show that the spin glass transition in the model
kagome antiferromagnet hydronium jarosite arises from a spin anisotropy. This
weaker energy scale is much smaller than that of the magnetic exchange, yet it
is responsible for the energy barriers that are necessary to stabilise a glassy
magnetic phase at finite temperature. The resultant glassy phase is quite
unlike those found in conventional disordered spin glasses as it is based on
complex collective rearrangements of spins called "spin folds". This simplifies
hugely theoretical treatment of both the complex dynamics characteristic of a
spin glass and the microscopic nature of the spin glass transition itself.
