Shocked in OP

UK Guardian-Observer's Dan Pearson, who writes about gardens and gardening,

was shocked to see these vast, low-slung Arts & Crafts family homes littering crisp expanses of lawn in the wealthy Oak Park area on the edge of [Chicago]. Not a prairie in sight, but impressive nonetheless.

Shocked meaning delighted? I think so.  He is writing about prairies as having a comeback:

A way with the prairies [subtitled:]

When intensive farming muscled in on the American midwest, vast wildernesses were trampled in the rush. Now, says Dan Pearson, they're rising again

So he is glad to see OP’s big lawns — contrasted, we know, by Hemingway with its narrow minds — as a good sign, in fact “impressive.”

Pearson refers to FL Wright and prairie style architecture as “developed in [Chicago] suburbs.”  But he “had only known [Wright’s] wonderful, iconic Fallingwater [in Western Pennsylvania], cantilevered over a cascade in woodland,” before seeing what any of us can see any day on Forest Avenue and elsewhere in OP & RF.  So he was “shocked.”  Meaning delighted.