  We study a parsimonious but non-trivial model of the latent limit order book
where orders get placed with a fixed displacement from a center price process,
i.e.\ some process in-between best bid and best ask, and get executed whenever
this center price reaches their level. This mechanism corresponds to the
fundamental solution of the stochastic heat equation with multiplicative noise
for the relative order volume distribution. We classify various types of
trades, and introduce the trading excursion process which is a Poisson point
process. This allows to derive the Laplace transforms of the times to various
trading events under the corresponding intensity measure. As a main
application, we study the distribution of order avalanches, i.e.\ a series of
order executions not interrupted by more than an $\varepsilon$-time interval,
which moreover generalizes recent results about Parisian options.
