How do I find the start and end of dips in data

Hi, hopefully this makes some sense.

I have averaged my data of a few hundred dips to one set.

There are two dips here, there is a bit of noise before the dip occurs;

I want MatLab to determine with a decent estimate where the start of the dip is (around 110 here) and the end (around 400). This then allows me to measure the width.

I would do this manually, but there is over 500 graphs.

Cheers for any help!