Absolutely, in terms of terminal effect. 45-70 will KIA any big game in N. America.
The only drawback to 45-70 is trajectory, Elk can require some long-ish shots and 45-70 has something of a "rainbow" trajectory compared to the medium bore magnums or even 30-06.
Pick a load and get comfortable with shot placement out to 200yds or so and you'll do fine.