"I was walking down the track one day........"

[Khriss]

....in the merry merry month of....wot that ???"

 

Hmmm, sandy soil bank. Literally hundreds of lil burrows, abt pencil size and all done on roughly same day -from degradation of spoil heaps.

 

 

[Khriss]

...all about 3-4cm depth on sample probing.k

[Woodworks]

Bumble bee nest? Never seen so many entrances though 

[eggsarascal]

We had similar at the end of our garden, t'was Wasps.

[Paul in the woods]

I would suggest some sort of solitary bee, at this time of year I'd guess ivy bee, if you have much ivy near by?

 

https://www.wildlifetrusts.org/wildlife-explorer/invertebrates/bees-wasps-and-ants/ivy-bee

[Khriss]

Bees were a first thought Paul, but never seen such a pronounced clustering. Must have been 200 plus in a few square metres, all on a day or two. Will mark it up as a Habitat site. K

[nepia]

Our neighbour has solitary bees that emerge in late summer from a sandy bank; it looks just like your pic when it happens.  Probably 200 or so in an area the size of a table tennis table.