<p>Only by the afterglow of the sunset could the hikers find their path to the campsite at [so late] an hour. No error</p>
<p>The answer was No error, but I wasn’t sure if so late an hour was idiomatically correct. I was thinking “at such late” may be better. </p>
<p>Could someone please explain this to me? </p>
<p>-Thank-you SATP</p>
<p>Both versions are acceptable. Just recognize that “so late” is an acceptable phrase to use. I don’t see how I can explain why “so late” is acceptable, but I can illustrate it :)! </p>
<p>“The Scholar” was delivered before two Societies at the University of Virginia so late as the year 1876.” - Ralph Waldo Emerson. </p>
<p>“Even so late as less than half a century ago this region was still a most attractive little rus in urbe.” - Emerson. </p>
<p>Hey, if Emerson uses it, it must be correct :)!</p>
<p>Thank-you 
</p>
<p>So they are interchangeable when used with time?</p>
<p>I mean I would definately say “Sorry for handing it in SO LATE!”, but not “Sorry for handing it in such late”. </p>
<p>But I guess to make the first quote idiomatically correct I’d have to say “at so late an hour”?</p>